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

[Eric Fiselier]

So I have a patch for LIT proper that does something like this. However I'm not sure it fits nicely into LIT. I'll put it up for review and get @ddunbar's opinion of it. It may just be best to handle this on our end.

http://reviews.llvm.org/D6255