clang-format: [JS] support fields with case/switch/default labels.

Summary:
`case:` and `default:` would normally parse as labels for a `switch` block.
However in TypeScript, they can be used in field declarations, e.g.:

    interface I {
      case: string;
    }

This change special cases parsing them in declaration lines to avoid wrapping
them.

Reviewers: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D36148

llvm-svn: 310070