[clang] [Clang] Allow raw string literals in C as an extension (PR #88265)

[Jan Svoboda via cfe-commits]

jansvoboda11 wrote:

I assume that @benlangmuir added the scanner unit-test to demonstrate the current behavior instead of trying to make sure it's preserved. I think making it so that the test passes (actually handles raw string literals) and updating the FIXME in `DependencyDirectivesScanner.cpp` should be fine. CC @Bigcheese

https://github.com/llvm/llvm-project/pull/88265