[C11] Improve the diagnostic when accessing a member of an atomic struct

Member access for an atomic structure or union is unconditional
undefined behavior (C11 6.5.2.3p5). However, we would issue a confusing
error message about the base expression not being a structure or union
type.

GCC issues a warning for this case. Clang now warns as well, but the
warning is defaulted to an error because the actual access is still
unsafe.

This fixes Issue 54563.