clang-format: [js] Support template strings.

Merge template strings (marked by backticks ``).
Do not format any contents of template strings.

Patch by Martin Probst. Thank you.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@230011 91177308-0d34-0410-b5e6-96231b3b80d8