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

Summary:
`getIdentifierInfo()` includes all keywords, whereas non-null assertion
operators should only be recognized after non-keywords or pseudo keywords.
Ideally this should list all tokens that clang-format recognizes as a keyword,
but that are pseudo or no keywords in JS. For the time being, just recognize
the specific bits users ran into (`namespace` in this case).

Reviewers: djasper

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D33182

llvm-svn: 303038