[clang] [clang][CodeGen] Guard ubsan checks with `llvm.allow.ubsan.check` (PR #87436)

[Vitaly Buka via cfe-commits]

vitalybuka wrote:

I will land, and address followup comments, if any.
It does not change behavior without -mllvm flags.

https://github.com/llvm/llvm-project/pull/87436