[clang] [clang][Lex] add -Wnonportable-include-path-separator (PR #186770)

[Nikita Popov via cfe-commits]

nikic wrote:

This introduces a compile-time regression: https://llvm-compile-time-tracker.com/compare.php?from=c146e9fce8b46f2495a57c8ce7696da4da9d2a13&to=9a1860c45a915328b392fa1af0bbc75dbe96af7f&stat=instructions:u

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