As mentionned in D97044, it is fine if users include <atomic> and then
include <stdatomic.h> -- we don't need to error out for that case.
Differential Revision: https://reviews.llvm.org/D125579
This reverts commits a30a7948d and 5d1c1a243, which broke the LLDB
data formatters tests because they build with modules in C++11 mode.
Differential Revision: https://reviews.llvm.org/D97044