clang-format: [JS] fix async parsing.

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

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D20737

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@271184 91177308-0d34-0410-b5e6-96231b3b80d8