[MinGW] Look for libc++ headers in a triplet prefixed path as well

This makes it consistent with libstdc++ and the other default
include directories.

If these headers are found in both locations and one isn't a
symlink to the other, this will cause errors due to libc++ headers
having wrapper headers for some standard C headers, wrappers that
do #include_next the actual one.

If the same libc++ standard C wrapper header exists in more than one
include directory before the real system one, the header include
guard will stop it from doing another #include_next to pick up the
real one, breaking things.

As this is a rather uncommon situation, this should be acceptable
and toolchain maintainers can adapt accordingly if necessary.

Also simplify some of the existing code with a local variable.

Differential Revision: https://reviews.llvm.org/D45500

llvm-svn: 329946