clang-format: [JS] fix template string width counting.

Summary:
Simply looking at the final text greatly simplifies the algorithm and also
fixes a reported issue. This requires duplicating the "actual encoding width"
logic, but that seems cleaner than the column acrobatics before.

Reviewers: djasper, bkramer

Subscribers: cfe-commits, klimek

Differential Revision: http://reviews.llvm.org/D20208

llvm-svn: 269747