[PATCH] [libcxx] Teach libcxx's lit configuration new ways to find lit.site.cfg

[Eric Fiselier]

I think this is the better direction to actually go. clang's lit.cfg does something similar to this but it uses `llvm-config` to find lit.site.cfg. I don't think changing LIT proper is the right way to go.

Pinging this patch.

http://reviews.llvm.org/D6255