clang-format: [JS] Support for EC6 arrow functions.

Before:
  var b = a.map((x) = > x + 1);

After:
  var b = a.map((x) => x + 1);

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209112 91177308-0d34-0410-b5e6-96231b3b80d8