[C++11] Remove LLVM_HAS_CXX11_TYPETRAITS now that it is a constant due

[C++11] Remove LLVM_HAS_CXX11_TYPETRAITS now that it is a constant due
to the build being C++11.

There is clearly still plenty of simplification than can be done here by
using standard type traits instead of rolling our own in many places.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@202586 91177308-0d34-0410-b5e6-96231b3b80d8