add relatedInfo to error message for 'await' used in non-async function

Author: uhyo

src/compiler/checker.ts

@@ -23087,7 +23087,20 @@ namespace ts {
             // Grammar checking
             if (produceDiagnostics) {
                 if (!(node.flags & NodeFlags.AwaitContext)) {
-                    grammarErrorOnFirstToken(node, Diagnostics.await_expression_is_only_allowed_within_an_async_function);
+                    // use of 'await' in non-async function
+                    const sourceFile = getSourceFileOfNode(node);
+                    if (!hasParseDiagnostics(sourceFile)) {
+                        const span = getSpanOfTokenAtPosition(sourceFile, node.pos);
+                        const diagnostic = createFileDiagnostic(sourceFile, span.start, span.length, Diagnostics.await_expression_is_only_allowed_within_an_async_function);
+                        const func = getContainingFunction(node);
+                        if (func) {
+                            Debug.assert((getFunctionFlags(func) & FunctionFlags.Async) === 0, "Enclosing function should never be an async function.");
+                            const relatedInfo = createDiagnosticForNode(func, Diagnostics.Did_you_mean_to_mark_this_function_as_async);
+                            addRelatedInfo(diagnostic, relatedInfo);
+
+                        }
+                        diagnostics.add(diagnostic);
+                    }
                 }
 
                 if (isInParameterInitializerBeforeContainingFunction(node)) {

src/compiler/diagnosticMessages.json

@@ -1035,6 +1035,10 @@
         "category": "Error",
         "code": 1355
     },
+    "Did you mean to mark this function as 'async'?": {
+        "category": "Error",
+        "code": 1356
+    },
 
     "Duplicate identifier '{0}'.": {
         "category": "Error",
@@ -2959,7 +2963,7 @@
         "category": "Error",
         "code": 4104
     },
-    
+
     "The current host does not support the '{0}' option.": {
         "category": "Error",
         "code": 5001