clang-format: [JS] Support ES6 destructuring assignments.

Before:
  var[a, b, c] = [1, 2, 3];

After:
  var [a, b, c] = [1, 2, 3];

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209113 91177308-0d34-0410-b5e6-96231b3b80d8