[clang] Respect the [[clang::unsafe_buffer_usage]] attribute for field and constructor initializers (PR #91991)

[Zequan Wu via cfe-commits]

ZequanWu wrote:

Confirmed that this change causes crash with -Wunsafe-buffer-usage: https://godbolt.org/z/vcv8d6sYr. 

My previously manual-reduced repro in https://github.com/llvm/llvm-project/pull/91991#issuecomment-2477311737 happens to crash before this change.

https://github.com/llvm/llvm-project/pull/91991