[Concurrency] Fix parsing bug async function types and labeled parameters.

Author: DougGregor

lib/Parse/ParseType.cpp

@@ -1195,8 +1195,10 @@ ParserResult<TypeRepr> Parser::parseTypeTupleBody() {
   if (EllipsisLoc.isInvalid())
     EllipsisIdx = ElementsR.size();
 
-  bool isFunctionType = Tok.isAny(tok::arrow, tok::kw_throws,
-                                  tok::kw_rethrows);
+  bool isFunctionType =
+      Tok.isAny(tok::arrow, tok::kw_throws, tok::kw_rethrows) ||
+      (shouldParseExperimentalConcurrency() &&
+       Tok.isContextualKeyword("async"));
 
   // If there were any labels, figure out which labels should go into the type
   // representation.

test/Parse/async-syntax.swift

@@ -5,6 +5,7 @@ func asyncGlobal2() async throws { }
 
 typealias AsyncFunc1 = () async -> ()
 typealias AsyncFunc2 = () async throws -> ()
+typealias AsyncFunc3 = (_ a: Bool, _ b: Bool) async throws -> ()
 
 func testTypeExprs() {
   let _ = [() async -> ()]()