clang-format: [JS] post-fix non-null assertion operator.

Summary:
Do not insert whitespace preceding the "!" postfix operator. This is an
incomplete fix, but should cover common usage.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D21204

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@272524 91177308-0d34-0410-b5e6-96231b3b80d8