Add super fast _IsSame trait for internal use.

Clang provides __is_same that doesn't produce any instantiations
and just returns a bool. It's a lot faster than using std::is_same

I'll follow up with a patch to actually start using it.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@364148 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed