[RequirementMachine] Inherited requirements collection should account for synthesized protocols

ClangImporter adds `SynthesizedProtocolAttr` to model inheritance
from `Sendable` on protocols imported from Objective-C.

Author: xedin

lib/AST/RequirementMachine/RequirementLowering.cpp

@@ -858,7 +858,7 @@ void swift::rewriting::expandDefaultRequirements(ASTContext &ctx,
 }
 
 /// Collect structural requirements written in the inheritance clause of an
-/// AssociatedTypeDecl or GenericTypeParamDecl.
+/// AssociatedTypeDecl, GenericTypeParamDecl, or ProtocolDecl.
 void swift::rewriting::realizeInheritedRequirements(
     TypeDecl *decl, Type type, bool shouldInferRequirements,
     SmallVectorImpl<StructuralRequirement> &result,
@@ -891,6 +891,17 @@ void swift::rewriting::realizeInheritedRequirements(
 
     realizeTypeRequirement(dc, type, inheritedType, loc, result, errors);
   }
+
+  // Also check for `SynthesizedProtocolAttr`s with additional constraints added
+  // by ClangImporter. This is how imported protocols are marked `Sendable`
+  // without changing their inheritance lists.
+  auto attrs = decl->getAttrs().getAttributes<SynthesizedProtocolAttr>();
+  for (auto attr : attrs) {
+    auto inheritedType = attr->getProtocol()->getDeclaredType();
+    auto loc = attr->getLocation();
+
+    realizeTypeRequirement(dc, type, inheritedType, loc, result, errors);
+  }
 }
 
 /// StructuralRequirementsRequest realizes all the user-written requirements

test/IDE/print_objc_concurrency_interface.swift

@@ -43,6 +43,9 @@ import _Concurrency
 // CHECK: @available(*, unavailable)
 // CHECK-NEXT: extension AuditedBoth : @unchecked Sendable {
 
+// CHECK-LABEL: public protocol SendableProtocol
+// CHECK-SAME: : Sendable
+
 // CHECK-LABEL: enum SendableEnum :
 // CHECK-SAME: @unchecked Sendable
 

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

@@ -225,6 +225,9 @@ SENDABLE @interface AuditedSendable : NSObject @end
 @interface AuditedNonSendable : NSObject @end
 NONSENDABLE SENDABLE @interface AuditedBoth : NSObject @end
 
+SENDABLE @protocol SendableProtocol @end
+@protocol SendableProtocolRefined <SendableProtocol> @end
+
 typedef NS_ENUM(unsigned, SendableEnum) {
   SendableEnumFoo, SendableEnumBar
 };