[C11] Allow casting to an _Atomic-qualified type

We were failing to strip off atomic qualification when forming the cast
destination type, but properly stripping off cvr qualification. Now we
accept atomic, qualified, or unqualified destination types.

Note: the semantics of the cast still drop the qualifier, so such a
cast does not result in an atomic rvalue.

Fixes https://github.com/llvm/llvm-project/issues/39596