[ssaf][UnsafeBufferUsage] Add support for extracting unsafe pointers from all kinds of contributors (#184899)
- Generalize the `-Wunsafe-buffer-usage` API for finding unsafe pointers
in all kinds of `Decl`s
- Add support in SSAF-based UnsafeBufferUsage analysis for extracting
from various contributors
- Mock implementation of HandleTranslationUnit
rdar://171735836
This is a follow-up PR to
https://github.com/llvm/llvm-project/pull/182941