clang-format: [JS] do not wrap before any `is`.

Summary:
`is` type annotations can occur at any nesting level. For example:

    function x() {
      return function y(): a is B { ... };
    }

Breaking before the `is` above breaks TypeScript parsing the code. This
change prevents the wrap.

Reviewers: krasimir

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D47193

llvm-svn: 332968