clang-format: use `pb` as a canonical raw string delimiter for google style

This updates the canonical text proto raw string delimiter to `pb` for Google style, moving codebases towards a simpler and more consistent style.

Also updates a behavior where the canonical delimiter was not applied for raw strings with empty delimiters detected via well-known enclosing functions that expect a text proto, effectively making the canonical delimiter more viral. This feature is not widely used so this should be safe and more in line with promoting the canonicity of the canonical delimiter.

Reviewed By: sammccall

Differential Revision: https://reviews.llvm.org/D97688