Since we now have a value for __cplusplus for c++14, teach libc++ about it

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210380 91177308-0d34-0410-b5e6-96231b3b80d8
Testing infastructure: A template for char_traits where all the functions are constexpr, and a comparison predicate which counts how many times it's been called.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@210381 91177308-0d34-0410-b5e6-96231b3b80d8