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

[Ziqing Luo via cfe-commits]

ziqingluo-90 wrote:

> > I added @ziqingluo-90 @jkorous-apple as reviewers since you've approved recent changes to this warning. Please let us know if you're the right reviewers for this and feel free to loop in more people if necessary.
> 
> Thank you @ivanaivanovska, the profiling and research you did is already valuable. Looks like there are a lot of changes. I will review them but it may take a while.

I know it's annoying, but could you split the change into smaller ones? It gonna be easier to review and with lower risk to break our downstream usage of the warning.

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