[clang] Optimize -Wunsafe-buffer-usage. (PR #124554)

[Ilya Biryukov via cfe-commits]

https://github.com/ilya-biryukov commented:

Thanks a lot for this!

I have left quite a few comments, mostly NITs.
A few things that really stood out and I think we should fix:
- increasing the number of macros and signatures we use seems like a bad trade-off. It's much better to have more parameters and ignore them in most cases, but only have a single macro to deal with as a return. The lower complexity of the code using the macros is really something we should optimize for.
- the code is using the ASTMatchers terminology despite switching away from it. I find it to be causing more confusion than good. We are really replacing matchers with AST traversal, so I left a few suggestions that go in the direction of switching from 
- some code has been moved around, making it a little hard to diff between the original and updated version. I suspect it would not compile otherwise, but I am happy to help find workarounds to keep the code compiling and keep every single function and class that reimplemented the original code at the same place. I think this should really help to ensure the code is equivalent and will make it easier to review.

https://github.com/llvm/llvm-project/pull/124554