[C11] Allow initialization of an atomic-qualified pointer from a null pointer constant

The relevant language rule from C11 is 6.5.16.1p1: "the left operand is
an atomic, qualified, or unqualified pointer, and the right is a null
pointer constant; or". We correctly handled qualified or unqualified
pointer types, but failed to handle atomic-qualified pointer types. Now
we look through the atomic qualification before testing the constraint
requirements.

Fixes https://github.com/llvm/llvm-project/issues/49563
Differential Revision: https://reviews.llvm.org/D148730