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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

I am definitely happy to review the code very carefully one more time and also commit to quickly revert or fix any discrepancies if anything happens downstream (would only need reproducers and such if we do miss something).
We definitely wants to enable this warning and are willing to make sure this optimization works out as intended.

This has been cooking for quite some time now and I do understand the concerns involved in landing a very large patch and the associated review costs. However, the alternative is likely going to take much-much longer and may result in more complicated commit history and quite a bit of work to get there (@ivanaivanovska has mentioned it above too).

@ziqingluo-90 is having a big commit a complete no-go for you? Do you have other ideas on how to split it?
We definitely want to make sure everyone is happy with the outcomes.

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