clang-format: [JS] do not break after declare namespace.

Summary:
See TypeScript grammar for tokens following 'declare':
https://github.com/Microsoft/TypeScript/blob/master/doc/spec.md#A.10

Additional minor change:
clang-format: [JS] Prevent ASI before const.

Reviewers: djasper

Subscribers: cfe-commits, klimek

Differential Revision: https://reviews.llvm.org/D26274

llvm-svn: 286468