[clang] Revert "[clang][Lex] add -Wnonportable-include-path-separator (#186770)" (PR #190975)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Given the Diagnostic is `DefaultIgnore`, you could check if it's enabled before doing any expensive check.
I'm fairly surprised by the regression though, `#include` are comparatively rare things in source files.

https://github.com/llvm/llvm-project/pull/190975