[Remote AST] Use anonymous context descriptor mangled names for private types.

When an anonymous context descriptor provides a mangled name, use that
mangled name to provide the private declaration name for its child context.
This allows us to resolve private type names correctly when the corresponding
anonymous context has its mangled name.

Fixes rdar://problem/38231646.

Author: DougGregor

include/swift/ABI/Metadata.h

@@ -2852,11 +2852,19 @@ struct TargetAnonymousContextDescriptor final
   /// recorded in the metadata.
   ConstTargetPointer<Runtime, char> getMangledName() const {
     if (!hasMangledName())
-      return nullptr;
+      return ConstTargetPointer<Runtime, char>();
 
     return this->template getTrailingObjects<MangledContextName>()->name;
   }
 
+  /// Retrieve a pointer to the mangled context name structure.
+  const MangledContextName *getMangledContextName() const {
+    if (!hasMangledName())
+      return nullptr;
+
+    return this->template getTrailingObjects<MangledContextName>();
+  }
+
 private:
   template<typename T>
   using OverloadToken =
@@ -2873,6 +2881,7 @@ struct TargetAnonymousContextDescriptor final
     return cd->getKind() == ContextDescriptorKind::Anonymous;
   }
 };
+using AnonymousContextDescriptor = TargetAnonymousContextDescriptor<InProcess>;
 
 /// A protocol descriptor.
 ///