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

[Hubert Tong via cfe-commits]

hubert-reinterpretcast wrote:

Handling for local variables (not initialized with `{}`). The performance impact of guaranteeing the extension is artificially reduced because the baseline code generation uses `memset` over the entire union when the size is large, but initializing the bytes after the first member is not mandated.

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