clang-format: [JS] support `interface` as a free standing identifier.

Summary:
`interface` can be used as a fee standing identifier in JavaScript/TypeScript.
This change uses the heuristic of whether it's followed by another identifier
as an indication.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: http://reviews.llvm.org/D19240

llvm-svn: 266789