[Sema] Fix reports on async when checking only the API availability

-check-api-availability-only limits availability checking to the API
only, ensuring that was will appear in the swiftinterface is sound but
without raising issues about implementation details. This patch ensures
it correctly ignore non-public async functions as they wouldn't appear
in the API.

rdar://86174644

Author: xymus

lib/Sema/TypeCheckAvailability.cpp

@@ -1710,6 +1710,9 @@ void TypeChecker::checkConcurrencyAvailability(SourceRange ReferenceRange,
   ASTContext &ctx = ReferenceDC->getASTContext();
   if (ctx.LangOpts.DisableAvailabilityChecking)
     return;
+
+  if (!shouldCheckAvailability(ReferenceDC->getAsDecl()))
+    return;
 
   auto runningOS =
     TypeChecker::overApproximateAvailabilityAtLocation(

test/Sema/api-availability-only-ok.swift

@@ -3,7 +3,7 @@
 
 // RUN: %empty-directory(%t)
 
-// RUN: %swiftc_driver -emit-module %s -target %target-cpu-apple-macosx10.15 -emit-module-interface -emit-module-interface-path %t/main.swiftinterface -enable-library-evolution -check-api-availability-only -verify-emitted-module-interface
+// RUN: %swiftc_driver -emit-module %s -target %target-cpu-apple-macosx10.14 -emit-module-interface -emit-module-interface-path %t/main.swiftinterface -enable-library-evolution -check-api-availability-only -verify-emitted-module-interface
 // RUN: %target-swift-frontend -typecheck-module-from-interface %t/main.swiftinterface
 
 // REQUIRES: OS=macosx
@@ -102,3 +102,10 @@ public struct Struct {
     newFunc()
   }
 }
+
+internal func asyncFunc() async -> InternalActor {
+    fatalError()
+}
+
+actor InternalActor {
+}

test/Sema/api-availability-only.swift

@@ -1,7 +1,7 @@
 /// Test that -check-api-availability-only skips what is expected while checking
 /// the module API and SPI.
 
-// RUN: %target-typecheck-verify-swift -module-name MyModule -target %target-cpu-apple-macosx10.15 -check-api-availability-only -enable-library-evolution
+// RUN: %target-typecheck-verify-swift -module-name MyModule -target %target-cpu-apple-macosx10.14 -check-api-availability-only -enable-library-evolution
 
 /// The flag -check-api-availability-only should reject builds going up to IR and further.
 // RUN: not %target-build-swift -emit-executable %s -g -o %t -emit-module -Xfrontend -check-api-availability-only 2>&1 | %FileCheck %s
@@ -132,3 +132,24 @@ public struct Struct {
 extension NewProto { // expected-error {{'NewProto' is only available in macOS 11.0 or newer}}
     public func foo() {}
 }
+
+func asyncFunc() async -> Bool {
+    fatalError()
+}
+
+// expected-note @+1 {{add @available attribute to enclosing}}
+public func publicAsyncFunc() async -> Bool { // expected-error {{concurrency is only available in macOS 10.15.0 or newer}}
+    fatalError()
+}
+
+// expected-note @+1 {{add @available attribute to enclosing}}
+@usableFromInline func usableFromInlineAsyncFunc() async -> Bool { // expected-error {{concurrency is only available in macOS 10.15.0 or newer}}
+    fatalError()
+}
+
+actor InternalActor {
+}
+
+// expected-note @+1 {{add @available attribute to enclosing}}
+public actor PublicActor { // expected-error {{concurrency is only available in macOS 10.15.0 or newer}}
+}