Unbork `std::memory_order` ABI.

Summary:
We need to pin the underlying type of C++20' `std::memory_order` to match the C++17 version. Anything less is an ABI break.

At the moment it's `unsigned` before C++20 and `int` after. Or if you're using `-fshort-enums` it's `unsigned char` before C++20 and `int` after.

This patch explicitly specifies the underlying type of the  C++20 `memory_order` to be w/e type the compiler would have chosen for the C++17 version.

Reviewers: mclow.lists, ldionne

Reviewed By: ldionne

Subscribers: jfb, jdoerfert, #libc, zoecarver

Differential Revision: https://reviews.llvm.org/D59063

llvm-svn: 355755