Auto merge of #113457 - davidtwco:lint-ctypes-issue-113436, r=oli-obk

lint/ctypes: fix `()` return type checks

Fixes #113436.

`()` is normally FFI-unsafe, but is FFI-safe when used as a return type. It is also desirable that a transparent newtype for `()` is FFI-safe when used as a return type.

In order to support this, when a type was deemed FFI-unsafe, because of a `()` type, and was used in return type - then the type was considered  FFI-safe. However, this was the wrong approach - it didn't check that the `()` was part of a transparent newtype! The consequence of this is that the presence of a `()` type in a more complex return type would make it the entire type be considered safe (as long as the `()` type was the first that the lint found) - which is obviously incorrect.

Instead, this logic is removed, and after [consultation with t-lang](rust-lang/rust#113436 (comment)), I've fixed the bugs and inconsistencies and  made `()` FFI-safe within types.

I also refactor a function, but that's not too exciting.

Author: bors