Suggest replacing 'async' with 'await' at call site

It's pretty easy to typo-replace 'await' with 'async' and get a
confusing error about 'async' not being in scope. This patch updates the
diagnostic to suggest replacing 'async' with 'await' at the call-sites
so that users aren't left scratching their heads.

lib/Sema/PreCheckExpr.cpp

@@ -519,7 +519,15 @@ Expr *TypeChecker::resolveDeclRefExpr(UnresolvedDeclRefExpr *UDRE,
         diag.highlight(UDRE->getSourceRange());
         typo->addFixits(diag);
       } else {
-        emitBasicError();
+        if (Name.isSimpleName("async")) {
+          auto diag = Context.Diags.diagnose(Loc,
+              diag::cannot_find_in_scope_corrected, Name,
+              /*isOperator=*/false, "await");
+          diag.highlight(UDRE->getSourceRange());
+          diag.fixItReplace(Loc, "await");
+        } else {
+          emitBasicError();
+        }
       }
 
       corrections.noteAllCandidates();

test/Concurrency/await_typo_correction.swift

@@ -0,0 +1,31 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -parse-as-library
+// REQUIRES: concurrency
+
+func asyncFunc() async throws {}
+
+func anotherAsyncFunc() async -> Int {
+  return 42
+}
+
+@main struct MyProgram {
+  static func main() async throws {
+    // expected-error@+7 {{cannot find 'async' in scope; did you mean 'await'?}}{{9-14=await}}
+    // expected-error@+6 {{consecutive statements on a line must be separated by ';'}}
+    // expected-error@+5 {{call is 'async' but is not marked with 'await'}}
+    // expected-error@+4 {{call can throw but is not marked with 'try'}}
+    // expected-note@+3 {{did you mean to use 'try'?}}
+    // expected-note@+2 {{did you mean to handle error as optional value?}}
+    // expected-note@+1 {{did you mean to disable error propagation?}}
+    try async asyncFunc()
+
+    // expected-error@+4 {{consecutive statements on a line must be separated by ';'}}
+    // expected-error@+3 {{cannot find 'async' in scope; did you mean 'await'?}}{{13-18=await}}
+    // expected-warning@+2 {{result of call to 'anotherAsyncFunc()' is unused}}
+    // expected-error@+1 {{call is 'async' but is not marked with 'await'}}
+    let _ = async anotherAsyncFunc()
+
+    // Don't emit a diagnostic here
+    async let foo = anotherAsyncFunc()
+    let _ = await foo
+  }
+}