clang-format: [JS] Understand line breaks in concatenated strings.

Before:
  var literal = 'hello ' + 'world';

After:
  var literal = 'hello ' +
                'world';

There is no reason to concatenated two string literals with a '+' unless
the line break is intended.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@209413 91177308-0d34-0410-b5e6-96231b3b80d8