[libc++] Bump _LIBCPP_STD_VER to the next expected C++ version

We've decided to use `_LIBCPP_STD_VER >= xy` while discussing to change the constexpr macros, so let's finally bump the version macro to match that.

Reviewed By: ldionne, Mordante, huixie90, #libc, avogelsgesang

Spies: avogelsgesang, libcxx-commits

Differential Revision: https://reviews.llvm.org/D133323