[clang] [MinGW] Improve detection of libstdc++ headers on Fedora

There's some variation in where different toolchain distributions
(and linux distributions) package the mingw sysroots - this is
so far handled by adding specific known subdirectory paths
to the include and lib directory lists.

There are multiple degrees of combinatorics involved here though;
the distros may use different locations such as
/usr/x86_64-w64-mingw32/include or
/usr/x86_64-w64-mingw32/sys-root/mingw/include.

So far, this setup has been treated as base=/usr, subdir=x86_64-w64-mingw32,
and the driver tries to add further subdirectories such as
<base>/<subdir>/include, <base>/<subdir>/sys-root/mingw/include.

When it comes to libstdc++ (and libc++), each of these come with
a large number of potential subdirectories. Instead of further
exploding the combinatorics another step by adding all combinations
of all paths, check whether <base>/<subdir>/sys-root/mingw/include
exists, and if it does, append that subpath into the subdir variable.

This allows finding libstdc++ headers in e.g.
/usr/x86_64-w64-mingw32/sys-root/mingw/include/c++/x86_64-w64-mingw32
on Fedora.

The same logic (where everything belonging to this target fits
under one expanded <subdir> path, with just /include and /lib
under it) doesn't seem to apply on Gentoo, where the includes
are found in <base>/<subdir>/usr/include while the libraries
are in <base>/<subdir>/mingw/lib (see
8e218026). But apparently
the libstdc++ headers aren't installed under
<base>/<subdir>/usr/include, so that path hierarchy quirk doesn't
need to be taken into account in AddClangCXXStdlibIncludeArgs.

Differential Revision: https://reviews.llvm.org/D138693