[Refactoring] Do not add "async" if function is already async

Convert Function to Async is available on an async function. It could be
useful to run this refactoring still, as it would attempt to convert any
completion-handler functions to their async alternatives. Keep allowing
this, but make sure not to re-add "async" to the function declaration.

Resolves rdar://82156720

Author: bnbarham

lib/IDE/Refactoring.cpp

@@ -6671,7 +6671,8 @@ class AsyncConverter : private SourceEntityWalker {
     addRange(MidStartLoc, MidEndLoc);
 
     // Third chunk: add in async and throws if necessary
-    OS << " async";
+    if (!FD->hasAsync())
+      OS << " async";
     if (FD->hasThrows() || TopHandler.HasError)
       // TODO: Add throws if converting a function and it has a converted call
       //       without a do/catch

test/refactoring/ConvertAsync/convert_function.swift

@@ -21,6 +21,17 @@ func simpleErr(arg: String) async throws -> String { }
 func simpleRes(arg: String, _ completion: @escaping (Result<String, Error>) -> Void) { }
 func simpleRes(arg: String) async throws -> String { }
 
+// RUN: %refactor-check-compiles -convert-to-async -dump-text -source-filename %s -pos=%(line+1):1 | %FileCheck -check-prefix=ALREADY-ASYNC %s
+func alreadyAsync() async {
+  simple {
+    print($0)
+  }
+}
+// ALREADY-ASYNC: func alreadyAsync() async {
+// ALREADY-ASYNC-NEXT: let val0 = await simple()
+// ALREADY-ASYNC-NEXT: print(val0)
+// ALREADY-ASYNC-NEXT: }
+
 // RUN: %refactor-check-compiles -convert-to-async -dump-text -source-filename %s -pos=%(line+1):1 | %FileCheck -check-prefix=NESTED %s
 func nested() {
   simple {