clang-format: [JS] fix `of` detection.

Summary:
`of` is only a keyword when after an identifier, but not when after
an actual keyword.

Before:
    return of (a, b, c);
After:
    return of(a, b, c);

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D43440

llvm-svn: 325489