[IRGen/Runtime] Anonymous context descriptors can (should be) generic.

Anonymous context descriptors were being treated as non-generic by
IRGen, which lead to problems for (file)private types within generic
types. Emit generic parameters and requirements for anonymous contexts
as well.

The runtime was mostly prepared for this, and the ABI already
accounted for it, so the runtime change is minor---it only affected
building a demangle tree from metadata.

Fixes rdar://problem/46853806.

Author: DougGregor

lib/IRGen/GenMeta.cpp

@@ -510,6 +510,7 @@ namespace {
 
     void layout() {
       super::layout();
+      asImpl().addGenericSignature();
     }
 
     ConstantReference getParent() {
@@ -522,7 +523,7 @@ namespace {
     }
 
     GenericSignature *getGenericSignature() {
-      return nullptr;
+      return DC->getGenericSignatureOfContext();
     }
 
     bool isUniqueDescriptor() {

stdlib/public/runtime/Demangle.cpp

@@ -46,6 +46,9 @@ swift::_buildDemanglingForContext(const ContextDescriptor *context,
     [&](const ContextDescriptor *context) -> NodePointer {
       if (demangledGenerics.empty())
         return nullptr;
+
+      if (context->getKind() == ContextDescriptorKind::Anonymous)
+        return nullptr;
 
       auto generics = context->getGenericContext();
       if (!generics)

test/IRGen/anonymous_context_descriptors.sil

@@ -0,0 +1,23 @@
+// RUN: %target-swift-frontend -emit-ir %s | %FileCheck %s
+
+import Builtin
+import Swift
+
+protocol P { }
+
+class Blah<T: P> {
+  private struct Inner<U: P> { }
+}
+
+// Anonymous descriptor
+// CHECK: @"$s29anonymous_context_descriptors4BlahC5Inner33{{.*}}MXX" =
+
+// Flags: anonymous (2) + generic (0x80) + unique (0x40)
+// CHECK-SAME: i32 194
+
+// Parent
+// CHECK-SAME: $s29anonymous_context_descriptors4BlahCMn
+
+// # generic header
+// CHECK-SAME: i16 2, i16 2
+// CHECK-SAME: i16 4, i16 0

test/Runtime/associated_type_demangle_private.swift

@@ -43,6 +43,10 @@ private protocol P2 {
   associatedtype A
 }
 
+private func getP2_A<T: P2>(_: T.Type) -> Any.Type {
+  return T.A.self
+}
+
 struct Bar: P2 {
   typealias A = Int
 }
@@ -55,4 +59,19 @@ AssociatedTypeDemangleTests.test("private protocols") {
   expectEqual("C2<Bar>", String(describing: C2<Bar>.self))
 }
 
+// rdar://problem/46853806
+class C3<T: P>: P2 {
+  fileprivate struct Inner<U: P> { }
+  fileprivate typealias A = Inner<T>
+}
+
+extension Int: P {
+  typealias A = Int
+}
+
+AssociatedTypeDemangleTests.test("generic anonymous contexts") {
+  expectEqual("Inner<Int>", String(describing: getP2_A(C3<Int>.self)))
+}
+
+
 runAllTests()