clang-format: [JS] for await, and fix a crash with for loops.

Summary:
The syntax is actually `for await (const x of y)` (d'oh).
This also fixes a crash for `for` tokens not followed by additional tokens.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D33329

llvm-svn: 303382