lint/ctypes: ext. abi fn-ptr in internal abi fn

[davidtwco]

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.