[CIR] Implement lowering of atomic-func ptrs (#199045)
This actually is a near-zero effort implementation, since other than an
assertion, we had everything correct. It isn't clear where that
assertion came from (other than an over abundance of caution?). It
doesn't exist in classic codegen, which always counts on the
fallthrough.
However, we ARE missing some ptr-auth work in that area, so a
MissingFeatures is added to be a placeholder for it.
