lint/ctypes: ext. abi fn-ptr in internal abi fn
Fixes#94223.
- In the improper ctypes lint, instead of skipping functions with internal ABIs, check that the signature doesn't contain any fn-ptr types with external ABIs that aren't FFI-safe.
- When computing the ABI for fn-ptr types, remove an `unwrap` that assumed FFI-safe types in foreign fn-ptr types.
- I'm not certain that this is the correct approach.
0ab38e95bb