[clang] Reland: "[Frontend][PCH]-Add support for ignoring PCH options (-ignore-pch). (#142409)" (PR #143614)

[Matheus Izvekov via cfe-commits]

https://github.com/mizvekov approved this pull request.

LGTM.

So what changed since the last commit is that preprocessing-only mode doesn't imply the semantics of this flag anymore.

I think It's good practice to mention what changed since original landing.

https://github.com/llvm/llvm-project/pull/143614