[clang] [MinGW] Consider the per-target libc++ include directory too

The existing logic for per-target libc++ include directories only
seem to exist for the Gnu and Fuchsia drivers, added in
ea12d779 / D89013.

This is less generic than the corresponding case in the Gnu driver,
but matches the existing level of genericity in the MinGW driver
(and others too).

Differential Revision: https://reviews.llvm.org/D107893