[clang] [clang] Avoid memcopy for small structure with padding under -ftrivial-auto-var-init (PR #71677)

[via cfe-commits]

serge-sans-paille wrote:

> @serge-sans-paille those test failures were also highlighted by the precommit checks, would be good to check those before landing

I did validate locally, but I forgot I had some targets disabled :-/

https://github.com/llvm/llvm-project/pull/71677