[PATCH] [libcxx] Take advantage of gcc4.7 frontend support in type_traits

[Marshall Clow]

Thinking about this, I want to retract my previous suggestion.

This patch fixes some things, and does not increase the coupling between type_traits and gcc.

Lets commit this patch as-is, and then decouple the #ifdefs in another patch.

So, LGTM.

http://reviews.llvm.org/D4340