Make the Linux support for finding libc++ somewhat less braindead.

Now instead of just looking in the system root for it, we also look
relative to the clang binary's directory. This should "just work" in
almost all cases. I've added test cases accordingly.

This is probably *very* worthwhile to backport to the 3.4 branch so that
folks can check it out, build it, and use that as their host compiler
going forward.