clang-format: [JS] generator and async functions.

clang-format: [JS] generator and async functions.

For generators, see:
https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_generators
async functions are not quite in the spec yet, but stage 3 and already widely used:
http://tc39.github.io/ecmascript-asyncawait/

Reviewers: djasper

Subscribers: klimek

Differential Revision: http://reviews.llvm.org/D19204

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@267368 91177308-0d34-0410-b5e6-96231b3b80d8