Add fix recommendation

This adds a note with a possible way to fix the issue, recommending
removing the `async` effect and wrapping the body of the function in a
task.

Author: etcwilde

include/swift/AST/DiagnosticsSema.def

@@ -1451,6 +1451,8 @@ NOTE(fixit_rename_in_swift,none,
      "change Swift name to %0", (DeclName))
 NOTE(fixit_rename_in_objc,none,
      "change Objective-C selector to %0", (ObjCSelector))
+NOTE(remove_async_add_task,none,
+     "remove 'async' and wrap in 'Task' to use concurrency in %0", (DeclName))
 ERROR(no_objc_tagged_pointer_not_class_protocol,none,
       "@unsafe_no_objc_tagged_pointer can only be applied to class protocols",
       ())

lib/Sema/TypeCheckAttr.cpp

@@ -504,6 +504,9 @@ validateIBActionSignature(ASTContext &ctx, DeclAttribute *attr,
   if (FD->isAsyncContext()) {
     ctx.Diags.diagnose(FD->getAsyncLoc(), diag::attr_decl_async,
         attr->getAttrName(), FD->getDescriptiveKind());
+
+    ctx.Diags.diagnose(FD->getAsyncLoc(), diag::remove_async_add_task,
+                       FD->getName());
     valid = false;
   }
 

test/attr/attr_ibaction.swift

@@ -43,8 +43,11 @@ class IBActionWrapperTy {
   func evenMoreMagic(_: AnyObject) -> () {} // no-warning
 
   @available(macOS 10.15, *)
-  @IBAction // expected-error@+1 {{@IBAction instance method cannot be async}}
+  @IBAction
   func asyncIBAction(_: AnyObject) async -> () {}
+  // expected-error@-1 {{@IBAction instance method cannot be async}}
+  // expected-note@-2 {{remove 'async' and wrap in 'Task' to use concurrency in 'asyncIBAction'}}
+
 }
 
 struct S { }