RHEL: Look in more places to find g++ headers and runtime.

Some distros with ten years of support ship an old gcc but later offer
more recent versions for installation in parallel. These versions are
typically not only needed for the compilation of llvm/clang, but also to
properly use the clang binary that comes out.

Clang already searches /usr at runtime for the most recent installation
of gcc. This patch appends paths for add-on installations of gcc in
RHEL.

Patch by Michael Lampe.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@268912 91177308-0d34-0410-b5e6-96231b3b80d8