[clang] [clang][CodeGen] Zero init unspecified fields in initializers in C (PR #97121)

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

(There's basically no benefit to just directly replacing a memset with an equivalent store; most relevant LLVM optimizations are memset-aware.)

https://github.com/llvm/llvm-project/pull/97121