We already have support for using c++ headers from a custom location.

Before this patch we would still link with the system libstdc++. It worked
fine most of the time, but would break if the used headers were a lot newer
than the system libraries.

This patch changes the driver to use the libraries corresponding to the
headers the user selected.

This fixes, for example, using 4.5 headers in a system with gcc 4.1.

llvm-svn: 132497