[Syntax] Fix grammar and parsing for 'async'.

I had used the wrong kind of token for the contextual 'async' keyword
(consistently), as well as flipping the order of async/throws when
creating syntax nodes in the parser. Add a `-verify-syntax-tree` test
with valid syntax to ensure that we don't make this mistake again.

Problem found by Nate Chandler, thanks!

Author: DougGregor

lib/Parse/ParseType.cpp

@@ -465,10 +465,10 @@ ParserResult<TypeRepr> Parser::parseType(Diag<> MessageID,
       ParsedFunctionTypeSyntaxBuilder Builder(*SyntaxContext);
       Builder.useReturnType(std::move(*SyntaxContext->popIf<ParsedTypeSyntax>()));
       Builder.useArrow(SyntaxContext->popToken());
-      if (asyncLoc.isValid())
-        Builder.useAsyncKeyword(SyntaxContext->popToken());
       if (throwsLoc.isValid())
         Builder.useThrowsOrRethrowsKeyword(SyntaxContext->popToken());
+      if (asyncLoc.isValid())
+        Builder.useAsyncKeyword(SyntaxContext->popToken());
 
       auto InputNode(std::move(*SyntaxContext->popIf<ParsedTypeSyntax>()));
       if (auto TupleTypeNode = InputNode.getAs<ParsedTupleTypeSyntax>()) {

test/Parse/async-syntax.swift

@@ -0,0 +1,12 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency -verify-syntax-tree
+
+func asyncGlobal1() async { }
+func asyncGlobal2() async throws { }
+
+typealias AsyncFunc1 = () async -> ()
+typealias AsyncFunc2 = () async throws -> ()
+
+func testTypeExprs() {
+  let _ = [() async -> ()]()
+  let _ = [() async throws -> ()]()
+}

utils/gyb_syntax_support/DeclNodes.py

@@ -76,7 +76,8 @@
     Node('FunctionSignature', kind='Syntax',
          children=[
              Child('Input', kind='ParameterClause'),
-             Child('AsyncKeyword', kind='ContextualKeywordToken',
+             Child('AsyncKeyword', kind='IdentifierToken',
+                   classification='Keyword',
                    text_choices=['async'], is_optional=True),
              Child('ThrowsOrRethrowsKeyword', kind='Token',
                    is_optional=True,

utils/gyb_syntax_support/ExprNodes.py

@@ -191,7 +191,8 @@
     # NOTE: This appears only in SequenceExpr.
     Node('ArrowExpr', kind='Expr',
          children=[
-             Child('AsyncKeyword', kind='ContextualKeywordToken',
+             Child('AsyncKeyword', kind='IdentifierToken',
+                   classification='Keyword',
                    text_choices=['async'], is_optional=True),
              Child('ThrowsToken', kind='ThrowsToken',
                    is_optional=True),

utils/gyb_syntax_support/TypeNodes.py

@@ -167,8 +167,9 @@
              Child('Arguments', kind='TupleTypeElementList',
                    collection_element_name='Argument'),
              Child('RightParen', kind='RightParenToken'),
-             Child('AsyncKeyword', kind='Token', text_choices=['async'],
-                   is_optional=True),
+             Child('AsyncKeyword', kind='IdentifierToken',
+                   classification='Keyword',
+                   text_choices=['async'], is_optional=True),
              Child('ThrowsOrRethrowsKeyword', kind='Token',
                    is_optional=True,
                    token_choices=[