[SILGen] Fix leak in objc async thunk.

Fix a leak when emitting the native to foreign thunk for an async
function which fulfills an Objective-C protocol requirement which can be
fulfilled with either a value or an error via a nullable completion.

Previously, the SIL in question used to look like this:

```sil
  %maybe_completion = ...
  try_apply %impl..., normal success, ...

success(%value):
  switch_enum %maybe_completion...
      case some!enumelt: invoke
      case none!enumelt: ignore

ignore:
  br join

invoke(%completion):
  %some_value = enum Optional, some!enumelt, %value // consumes %value
  %guaranteed_some_value = begin_borrow %some_value
  %none_error = enum Optional, none!enumelt
  apply %completion(%guaranteed_some_value, %none_error)
  end_borrow %guaranteed_some_value
  destroy_value %some_value
  br join

join:
  destroy_value %maybe_completion
  ...
```

which leaks %value on the codepath through `ignore`.

Note that `%value` is consumed by the `enum` instruction, but
`%completion` is invoked with `%guaranteed_some_value`, a guaranteed
value.  So there is no need to consume %value in `invoke`.

Here, `%value` itself is borrowed and forwarded into an enum instruction
whose result is passed to `%completion`:

```sil
  %maybe_completion = ...
  try_apply %impl..., normal success, ...

success(%value):
  switch_enum %maybe_completion...
      case some!enumelt: invoke
      case none!enumelt: ignore

ignore:
  br join

invoke(%completion):
  %guaranteed_value = begin_borrow %value
  %guaranteed_some_value = enum Optional, some!enumelt, %guaranteed_value
  %none_error = enum Optional, none!enumelt
  apply %completion(%guaranteed_some_value, %none_error)
  end_borrow %guaranteed_some_value
  br join

join:
  destroy_value %maybe_completion
  destroy_value %value
  ...
```

Because an argument scope was already being created and a cleanup was
already being pushed for `%value`, nothing more is required to fix the
issue than to reorder the enum and the borrow.

rdar://119732084

Author: nate-chandler

lib/SILGen/SILGenBridging.cpp

@@ -1773,10 +1773,10 @@ void SILGenFunction::emitNativeToForeignThunk(SILDeclRef thunk) {
                        SILParameterInfo param) {
       auto bridgedTy = param.getInterfaceType();
       auto bridgedArg = emitNativeToBridgedValue(loc,
-                                 arg, nativeFormalTy,
+                                 arg.borrow(*this, loc), nativeFormalTy,
                                  bridgedTy,
                                  SILType::getPrimitiveObjectType(bridgedTy));
-      completionHandlerArgs.push_back(bridgedArg.borrow(*this, loc).getValue());
+      completionHandlerArgs.push_back(bridgedArg.getValue());
     };
 
     Scope completionArgDestructureScope(*this, loc);

test/Inputs/clang-importer-sdk/usr/include/ObjCConcurrency.h

@@ -82,6 +82,7 @@ typedef void (^NonsendableCompletionHandler)(NSString * _Nullable, NSString * _N
 -(void)doSomethingSlowNullably:(NSString *)operation completionHandler:(void (^ _Nullable)(NSInteger))handler;
 -(void)findAnswerNullably:(NSString *)operation completionHandler:(void (^ _Nullable)(NSString *))handler;
 -(void)doSomethingDangerousNullably:(NSString *)operation completionHandler:(void (^ _Nullable)(NSString *_Nullable, NSError *_Nullable))handler;
+-(void)doSomethingUnspecifiedNullablyWithCompletionHandler:(void (^ _Nullable)(NSString *_Nullable, NSError *_Nullable))handler;
 
 // rdar://72604599
 - (void)stopRecordingWithHandler:(nullable void (^)(NSObject *_Nullable_result x, NSError *_Nullable error))handler __attribute__((swift_async_name("stopRecording()"))) __attribute__((swift_async(not_swift_private, 1)));

test/SILGen/objc_async_from_swift.swift

@@ -127,6 +127,13 @@ class SlowServerlet: SlowServer {
         return x
     }
 
+    // CHECK-LABEL: sil{{.*}}13SlowServerlet{{.*}}30doSomethingUnspecifiedNullably{{.*}} :
+    // CHECK:         [[GENERIC_EXECUTOR:%.*]] = enum $Optional<Builtin.Executor>, #Optional.none
+    // CHECK:         hop_to_executor [[GENERIC_EXECUTOR]] :
+    override func doSomethingUnspecifiedNullably() async throws -> String {
+      fatalError()
+    }
+
     // CHECK-LABEL: sil{{.*}}13SlowServerlet{{.*}}17doSomethingFlaggy{{.*}} :
     // CHECK:         [[GENERIC_EXECUTOR:%.*]] = enum $Optional<Builtin.Executor>, #Optional.none
     // CHECK:         hop_to_executor [[GENERIC_EXECUTOR]] :

validation-test/SILGen/rdar119732084.swift

@@ -0,0 +1,67 @@
+// RUN: %empty-directory(%t)
+// RUN: split-file %s %t
+// RUN: %target-swift-emit-silgen -module-name Conformance -I %t %t/Conformance.swift -disable-availability-checking | %FileCheck %s
+
+// REQUIRES: concurrency
+// REQUIRES: objc_interop
+
+//--- module.modulemap
+module ObjCProto {
+  header "objc_proto.h"
+  export *
+}
+
+//--- objc_proto.h
+#import <Foundation/Foundation.h>
+
+@protocol Doable <NSObject>
+
+- (void)doitWithCompletion:(void (^ __nullable)(NSObject * __nullable result, NSError * __nullable error))completion;
+
+@end
+
+//--- Conformance.swift
+import ObjCProto
+
+public final class ConformsToProto: NSObject {}
+
+extension ConformsToProto: Doable {
+// CHECK-LABEL: sil shared [thunk] [ossa] @$s11Conformance15ConformsToProtoC4doitSo8NSObjectCyYaKFyyYacfU_To : {{.*}} {
+// CHECK:       {{bb[0-9]+}}([[UNOWNED_MAYBE_COMPLETION:%[^,]+]] :
+// CHECK:         [[MAYBE_COMPLETION:%[^,]+]] = copy_block [[UNOWNED_MAYBE_COMPLETION]]
+// CHECK:         try_apply {{.*}} normal [[SUCCESS:bb[0-9]+]], error [[FAILURE:bb[0-9]+]]
+// CHECK:       [[SUCCESS]]([[RESULT:%[^,]+]] :
+// CHECK:         [[GUARANTEED_MAYBE_COMPLETION_SUCCESS:%[^,]+]] = begin_borrow [[MAYBE_COMPLETION]]
+// CHECK:         switch_enum [[GUARANTEED_MAYBE_COMPLETION_SUCCESS]]
+// CHECK-SAME:        case #Optional.some!enumelt: [[SUCCESS_AND_COMPLETION:bb[0-9]+]]
+// CHECK-SAME:        case #Optional.none!enumelt: [[SUCCESS_NO_COMPLETION:bb[0-9]+]]
+// CHECK:       [[SUCCESS_NO_COMPLETION]]:
+// CHECK:         br [[SUCCESS_JOIN:bb[0-9]+]]
+// CHECK:       [[SUCCESS_AND_COMPLETION]]
+// CHECK:         br [[SUCCESS_JOIN]]
+// CHECK:       [[SUCCESS_JOIN]]:
+// CHECK:         end_borrow [[GUARANTEED_MAYBE_COMPLETION_SUCCESS]]
+// CHECK:         destroy_value [[MAYBE_COMPLETION]]
+// CHECK:         destroy_value [[RESULT]]
+// CHECK:         br [[EXIT:bb[0-9]+]]
+// CHECK:       [[FAILURE]]([[ERROR:%[^,]+]] :
+// CHECK:         [[GUARANTEED_MAYBE_COMPLETION_FAILURE:%[^,]+]] = begin_borrow [[MAYBE_COMPLETION]]
+// CHECK:         switch_enum [[GUARANTEED_MAYBE_COMPLETION_FAILURE]]
+// CHECK-SAME:        case #Optional.some!enumelt: [[FAILURE_AND_COMPLETION:bb[0-9]+]]
+// CHECK-SAME:        case #Optional.none!enumelt: [[FAILURE_NO_COMPLETION:bb[0-9]+]]
+// CHECK:       [[FAILURE_NO_COMPLETION]]:
+// CHECK:         br [[FAILURE_JOIN:bb[0-9]+]]
+// CHECK:       [[FAILURE_AND_COMPLETION]]
+// CHECK:         br [[FAILURE_JOIN]]
+// CHECK:       [[FAILURE_JOIN]]:
+// CHECK:         end_borrow [[GUARANTEED_MAYBE_COMPLETION_FAILURE]]
+// CHECK:         destroy_value [[MAYBE_COMPLETION]]
+// CHECK:         destroy_value [[ERROR]]
+// CHECK:         br [[EXIT]]
+// CHECK:       [[EXIT]]:
+// CHECK-LABEL: } // end sil function '$s11Conformance15ConformsToProtoC4doitSo8NSObjectCyYaKFyyYacfU_To'
+  public func doit() async throws -> NSObject {
+    fatalError()
+  }
+}
+