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

[Sam James via cfe-commits]

thesamesam 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.

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