fix bug with unidentified custom attribute during serialization

In Swift 5 mode, I only warned about the global-actor attribute
becoming unnecessary in the future, yet I was still returning
None for the global-actor attribute checking request. This led
to the attribute remaining unidentified, but also not removed.
During module serialization, this problem was manifesting as
emitting a typeless attribute, which when deserialized would
trigger a segfault.

Author: kavon

lib/Sema/TypeCheckConcurrency.cpp

@@ -360,11 +360,15 @@ GlobalActorAttributeRequest::evaluate(
       if (!var->isStatic() && var->isOrdinaryStoredProperty()) {
         if (auto *nominal = var->getDeclContext()->getSelfNominalTypeDecl()) {
           if (nominal->isValueType()) {
+
             var->diagnose(diag::global_actor_on_storage_of_value_type,
                           var->getName(), nominal->getDescriptiveKind())
               .highlight(globalActorAttr->getRangeWithAt())
               .warnUntilSwiftVersion(6);
-            return None;
+
+            // In Swift 6, once the diag above is an error, it is disallowed.
+            if (var->getASTContext().isSwiftVersionAtLeast(6))
+              return None;
           }
         }
       }

test/Concurrency/Inputs/dynamically_replaceable.swift

@@ -0,0 +1,15 @@
+
+// dynamically_replaceable
+@MainActor
+public dynamic func dynamicOnMainActor() { }
+
+// property wrapper + main actor
+@propertyWrapper
+public struct IntWrapper {
+
+  public init(wrappedValue: Int) {
+    self.wrappedValue = wrappedValue
+  }
+
+  @MainActor public var wrappedValue: Int
+}

test/Concurrency/Inputs/other_global_actor_inference.swift

@@ -1,8 +1,8 @@
 // RUN: %empty-directory(%t)
-// RUN: %target-swift-frontend -emit-module -emit-module-path %t/dynamically_replaceable.swiftmodule -module-name dynamically_replaceable -warn-concurrency %S/Inputs/dynamically_replaceable.swift
+// RUN: %target-swift-frontend -emit-module -emit-module-path %t/other_global_actor_inference.swiftmodule -module-name other_global_actor_inference -warn-concurrency %S/Inputs/other_global_actor_inference.swift
 // RUN: %target-typecheck-verify-swift -I %t -disable-availability-checking
 // REQUIRES: concurrency
-import dynamically_replaceable
+import other_global_actor_inference
 
 actor SomeActor { }
 
@@ -31,6 +31,14 @@ struct GenericGlobalActor<T> {
 @MainActor class Copper {}
 @MainActor func iron() {}
 
+struct Carbon {
+  @IntWrapper var atomicWeight: Int
+
+  func getWeight() -> Int {
+    return atomicWeight
+  }
+}
+
 // ----------------------------------------------------------------------
 // Check that @MainActor(blah) doesn't work
 // ----------------------------------------------------------------------