The underlying type of an enumeration is the non-atomic, unqualified
version of the specified type. Clang was rejecting such enumerations,
with a hard error, but now has the ability to downgrade the error into a
warning. Additionally, we diagnose (as a warning) dropping other
qualifiers. _Atomic is special given that an atomic type need not have
the same size as its non-atomic counterpart, and that the C++ version
of <stdatomic.h> defines _Atomic to std::atomic for easing cross-
language atomic use and std::atomic is an invalid enum base in C++.
(Note: we expose _Atomic in C++ even without including
<stdatomic,h>.)

Fixes #147736