clang-format: [JS] handle `as const`.

Summary:
TypeScript 3.4 supports casting into a const type using `as const`:

    const x = {x: 1} as const;

Previously, clang-format would insert a space after the `const`. With
this patch, no space is inserted after the sequence `as const`.

Reviewers: krasimir

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66736

llvm-svn: 369916