[clang] Clang: Return new layout path if cannot find CRT (PR #87319)

[Fangrui Song via cfe-commits]

https://github.com/MaskRay requested changes to this pull request.

I don't think we should add another copy of `buildCompilerRTBasename` to `getCompilerRT` to make it more complex. Let's keep the discussion in #87150

https://github.com/llvm/llvm-project/pull/87319