clang-format: [JS] allow breaking after non-null assertions.

Summary:
Previously clang-format would not break after any !. However in TypeScript, ! can be used as a post fix operator for non-nullability:
    x.foo()!.bar()!;

With this change, clang-format will wrap after the ! if it is likely a post-fix non null operator.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D30705

llvm-svn: 297606