[clang] [clang][scandeps] Improve handling of rawstrings. (PR #139504)

[Tobias Hieta via cfe-commits]

tru wrote:

Sorry for the delay. I have addressed both your comments now @benlangmuir - reworked it to use Lexer::getEscapedNewLineSize() and removed the assertline. I also expanded the test coverage.

https://github.com/llvm/llvm-project/pull/139504