[libc++] Make std::allocator_arg and friends conforming in C++17

[libc++] Make std::allocator_arg and friends conforming in C++17

This patch makes global tag variables like std::allocator_arg
conform to C++17 by defining them as inline constexpr variables.
This is possible without creating an ODR violation now that we don't
define strong definitions of those variables in the shared library
anymore.

Differential Revision: https://reviews.llvm.org/D145589