[clang] [Clang] Optimize -Wunsafe-buffer-usage. (PR #125492)

[Ilya Biryukov via cfe-commits]

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

We really want this to enable the `Wunsafe-buffer-usage` more broadly in our codebase.

Right now the performance impact is just way too high for us.
So thanks a lot for working on this!

Obviously, we need to add people who own the particular piece of code before landing this. I will also make another round to double-check all matcher rewrites and ensure they're equivalent (it's a bit hard to do in one go, I haven't found any discrepancies just yet).

PS #124554 has some previous discussions, but was closed for technical reasons.

https://github.com/llvm/llvm-project/pull/125492