[clang-format] Do not format raw string literals inside a recognized function...

[clang-format] Do not format raw string literals inside a recognized function with a non-recognized delimiter

Summary:
This stops clang-format from touching raw string contents with unrecognized delimiters inside recognized functions.
Unrecognized delimiters signal that the string might be special.

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D48728

llvm-svn: 335876