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.
Well, I dunno how MinGW works, so can't judge that part. Furthermore, I'm thoroughly confused why this works on top of existing (non-Gentoo?) test tree. Could you explain it a bit more?
It looks like the clang driver just blindly adds these directories to the list of include paths, regardless of whether they exist or not. So with the current test, they don't need to exist (but maybe it would be good to create such empty directories to make the test a bit more correct, in case the driver is modified to not add nonexistent directories blindly).