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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@303382 91177308-0d34-0410-b5e6-96231b3b80d8