[clang] Revert "[clang] [MinGW] Fix paths on Gentoo" (PR #219979)

[Zhongteng Gui via cfe-commits]

dragon-archer wrote:

> > Gentoo no longer uses mingw/lib and include/g++-vXX, see [packages.gentoo.org/packages/dev-util/llvm-mingw64](https://packages.gentoo.org/packages/dev-util/llvm-mingw64) and [packages.gentoo.org/packages/dev-util/mingw64-toolchain](https://packages.gentoo.org/packages/dev-util/mingw64-toolchain).
> 
> I'm not sure that's right. Those packages are for convenience but different paths may be used with crossdev.

I'm not 100% sure, but from [here](https://www.portagefilelist.de/index.php?p=package&id=f0f537481dd90bbcd623fe3918da5558), it seemes that Gentoo's crossdev no longer supports MinGW?

BTW, even if Gentoo still uses these non-standard paths, I think it'd be better to keep the original patch as Gentoo-specific, instead of applying it here, so that other MinGW platforms won't be affected.

https://github.com/llvm/llvm-project/pull/219979