[clang] [clang][Lex] warn on backslashes in include paths (PR #186770)

[Rose Hudson via cfe-commits]

rosefromthedead wrote:

Assuming we do indeed apply the first fix and skip the others: I suppose it's not the end of the world to have to run -fixit twice, and having backslashes be accepted by default would make this cleaner. I'll try that out.

https://github.com/llvm/llvm-project/pull/186770