[clang] [MinGW] Fix paths on Gentoo

There's code in clang/lib/Driver/ToolChains/Gnu.cpp for Clang to use Gentoo's include and lib paths, but this is missing for mingw, meaning that any C++ programs using the STL will fail to compile.

See https://bugs.gentoo.org/788430

Differential Revision: https://reviews.llvm.org/D111081