[Concurrency] Emit async methods in ObjC protocols once.

Previously, the method table would contain duplicate copies due to the
ProtocolDecl carrying a completionHandler-based version of the method,
as well as the async version of the method.

Fixes rdar://76192003.

Author: varungandhi-apple

lib/IRGen/GenClass.cpp

@@ -1153,8 +1153,19 @@ namespace {
         Protocols.push_back(proto);
       }
 
-      for (Decl *member : theProtocol->getMembers())
+      for (Decl *member : theProtocol->getMembers()) {
+        // Async methods coming from ObjC protocols shouldn't be recorded twice.
+        // At the moment, the language doesn't allow suppressing the
+        // completionHandler-based variant, so this is sufficient.
+        if (theProtocol->hasClangNode() && theProtocol->isObjC()) {
+          if (auto funcOrAccessor = dyn_cast<AbstractFunctionDecl>(member)) {
+            if (funcOrAccessor->isAsyncContext()) {
+              continue;
+            }
+          }
+        }
         visit(member);
+      }
     }
 
     /// Gather protocol records for all of the explicitly-specified Objective-C

test/Concurrency/Inputs/Delegate.h

@@ -17,5 +17,8 @@
 
 @end
 
+@protocol MyAsyncProtocol
+-(void)myAsyncMethod:(void (^ _Nullable)(NSError * _Nullable, NSString * _Nullable))completionHandler;
+@end
 
 #endif

test/Concurrency/objc_async_protocol_irgen.swift

@@ -0,0 +1,15 @@
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -enable-experimental-concurrency -import-objc-header %S/Inputs/Delegate.h %s -emit-ir -o - | %FileCheck %s
+// REQUIRES: concurrency
+// REQUIRES: objc_interop
+
+
+let anyObject: AnyObject = (MyAsyncProtocol.self as AnyObject) // or something like this
+
+// rdar://76192003
+// Make sure we don't emit 2 copies of methods, due to a completion-handler
+// version and another due to an async based version.
+
+// CHECK-LABEL: @_PROTOCOL_INSTANCE_METHODS_MyAsyncProtocol = internal constant
+// CHECK-SAME: selector_data(myAsyncMethod:)
+// CHECK-NOT: selector_data(myAsyncMethod:)
+// CHECK-SAME: align 8