clang-format: [JS] handle destructuring `of`.

Summary:
Previously, clang-format would drop a space character between `of` and
then following (non-identifier) token if the preceding token was part of
a destructuring assignment (`}` or `]`).

Before:
    for (const [a, b] of[]) {}

After:
    for (const [a, b] of []) {}

Reviewers: djasper

Subscribers: klimek

Differential Revision: https://reviews.llvm.org/D40411

llvm-svn: 318942