Headers: tweak inclusion condition for stdatomic.h

MSVC requires that C++23 be available (_HAS_CXX23) else the entire
content is elided. Conditionalise the inclusion properly so that C/C++
code using stdatomic.h for memory_order_* constants are able to do
so without changing the C++ standard. This repairs builds of Swift and
libdispatch after ba49d39b.

Differential Revision: https://reviews.llvm.org/D139266
Reviewed By: aaron.ballman, Mordante, fsb4000