clang-format: [JS] whitespace after async in arrow functions.

Summary:
Async arrow functions should be marked with a whitespace after the async keyword, before the parameter list:
    x = async () => foo();

Before:
    x = async() => foo();

This makes it easier to tell apart an async arrow function from a call to a function called async.

Reviewers: bkramer

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D30399

llvm-svn: 296330