[clang-format] Adds a canonical delimiter to raw string formatting

Summary:
This patch adds canonical delimiter support to the raw string formatting.
This allows matching delimiters to be updated to the canonical one.

Reviewers: bkramer

Reviewed By: bkramer

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D42187

llvm-svn: 322956