Fix await in effect with async

Typing `func foo() await {` is something that folks do from time-to-time.
The old error message was unintuitive and suggested adding a semicolon
between the parenthesis and the await, then proceeded to complain about
the opening brace. This wasn't very clear about what the error actually
was.

Pulling this in line with `try`, `throw`, and `throws`, this patch
suggests replacing `await` with `async` when in the function effect
position, and provides a nice fix-it.

lib/Parse/ParsePattern.cpp

@@ -926,6 +926,7 @@ bool Parser::isEffectsSpecifier(const Token &T) {
   //       'parseEffectsSpecifiers()'.
 
   if (T.isContextualKeyword("async") ||
+      (T.isContextualKeyword("await") && !T.isAtStartOfLine()) ||
       T.isContextualKeyword("reasync"))
     return true;
 
@@ -984,6 +985,13 @@ ParserStatus Parser::parseEffectsSpecifiers(SourceLoc existingArrowLoc,
       consumeToken();
       continue;
     }
+    // diagnose 'await'
+    if (Tok.isContextualKeyword("await") && !Tok.isAtStartOfLine()) {
+      diagnose(Tok, diag::await_in_function_type)
+        .fixItReplace(Tok.getLoc(), "async");
+      consumeToken();
+      continue;
+    }
 
     // 'throws'/'rethrows', or diagnose 'throw'/'try'.
     if (Tok.isAny(tok::kw_throws, tok::kw_rethrows) ||