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

[Dana Jansens via cfe-commits]

danakj wrote:

I have rebased (which dropped the previous PR's commits) and applied the requested changes in a second commit. I also fixed the test failures in that second commit.

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