Auto merge of #146273 - niacdoial:improperctypes-refactor2, r=petrochenkov

lint ImproperCTypes: refactor linting architecture (part 2)



This is the second PR in an effort to split #134697 (refactor plus overhaul of the ImproperCTypes family of lints) into individually-mergeable parts.

Contains the changes of the first PR, and splits the core type checking function into several bits, each focused on a specific aspect of FFI-safety.
Some logic which was outside of said core function was also moved into the new functions.

Superset of: #146271

Author: bors