clang-format: [JS] Basic support for escape sequences in regex literals.

Before:
  var regex = /\\/ g; // This isn't even recognized as regex.

After:
  var regex = /\\/g; // It now is.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@208539 91177308-0d34-0410-b5e6-96231b3b80d8