clang-format: [JS] fix async parsing.

[mprobst]

Summary

Only treat the sequence async function as the start of a function expression,
as opposed to every occurrence of the token async (whoops).

Also refactors AnnotatedLine.startsWith/endsWith by extracting the core functionality
into FormatToken.startsSequence/endsSequence. This allows checking tokens within
the pointered linked list structure with a lookahead, automatically ignoring
comments, which is useful in many places (e.g. see subsequent commit).

[mprobst]

• fix description

[djasper]

Looks good.

cfe/trunk/lib/Format/UnwrappedLineParser.cpp

case tok::identifier: {
       if (FormatTok->is(TT_MacroBlockEnd)) {
         addUnwrappedLine();
         return;
       }
 
       // Parse function literal unless 'function' is the first token in a line
       // in which case this should be treated as a free-standing function.
       if (Style.Language == FormatStyle::LK_JavaScript &&
-          FormatTok->isOneOf(Keywords.kw_async, Keywords.kw_function) &&
+          (FormatTok->is(Keywords.kw_function) ||
+           FormatTok->startsSequence(Keywords.kw_async,
+                                     Keywords.kw_function)) &&
           Line->Tokens.size() > 0) {
         tryToParseJSFunction();
         break;
       }
       if ((Style.Language == FormatStyle::LK_JavaScript ||
            Style.Language == FormatStyle::LK_Java) &&
           FormatTok->is(Keywords.kw_interface)) {
         if (Style.Language == FormatStyle::LK_JavaScript) {
do {
     } else {
       return false;
     }
   } while (!eof());
   return false;
 }
 
 void UnwrappedLineParser::tryToParseJSFunction() {
-  assert(FormatTok->isOneOf(Keywords.kw_async, Keywords.kw_function));
+  assert(FormatTok->is(Keywords.kw_function) ||
+         FormatTok->startsSequence(Keywords.kw_async, Keywords.kw_function));
   if (FormatTok->is(Keywords.kw_async))
     nextToken();
   // Consume "function".
   nextToken();
 
   // Consume * (generator function).
   if (FormatTok->is(tok::star))
     nextToken();

 bool UnwrappedLineParser::parseBracedList(bool ContinueOnSemicolons) {
   bool HasError = false;
   nextToken();
 
   // FIXME: Once we have an expression parser in the UnwrappedLineParser,
   // replace this by using parseAssigmentExpression() inside.
   do {
     if (Style.Language == FormatStyle::LK_JavaScript) {
-      if (FormatTok->isOneOf(Keywords.kw_async, Keywords.kw_function)) {
+      if (FormatTok->is(Keywords.kw_function) ||
+          FormatTok->startsSequence(Keywords.kw_async, Keywords.kw_function)) {
         tryToParseJSFunction();
         continue;
       }
       if (FormatTok->is(TT_JsFatArrow)) {
         nextToken();
         // Fat arrows can be followed by simple expressions or by child blocks
         // in curly braces.
         if (FormatTok->is(tok::l_brace)) {
case tok::l_brace:
       break;
     case tok::at:
       nextToken();
       if (FormatTok->Tok.is(tok::l_brace))
         parseBracedList();
       break;
     case tok::identifier:
       if (Style.Language == FormatStyle::LK_JavaScript &&
-          FormatTok->isOneOf(Keywords.kw_async, Keywords.kw_function))
+          (FormatTok->is(Keywords.kw_function) ||
+           FormatTok->startsSequence(Keywords.kw_async, Keywords.kw_function)))
         tryToParseJSFunction();
       else
         nextToken();
       break;
     default:
       nextToken();
       break;
     }

cfe/trunk/unittests/Format/FormatTestJS.cpp

verifyFormat("async function* f() {\n"
                "  yield fetch(x);\n"
                "}");
   verifyFormat("export async function f() {\n"
                "  return fetch(x);\n"
                "}");
   verifyFormat("class X {\n"
                "  async asyncMethod() { return fetch(1); }\n"
                "}");
+  verifyFormat("function initialize() {\n"
+               "  // Comment.\n"
+               "  return async.then();\n"
+               "}\n");
 }
 
 TEST_F(FormatTestJS, ArrayLiterals) {
   verifyFormat("var aaaaa: List<SomeThing> =\n"
                "    [new SomeThingAAAAAAAAAAAA(), new SomeThingBBBBBBBBB()];");
   verifyFormat("return [\n"
                "  aaaaaaaaaaaaaaaaaaaaaaaaaaa, bbbbbbbbbbbbbbbbbbbbbbbbbbb,\n"
                "  ccccccccccccccccccccccccccc\n"