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

[Eli Friedman via cfe-commits]

efriedma-quic wrote:

There are two possible alternative sequences we could generate there:

```
int test(int x, int y) {
    struct S s;
    __builtin_memset(&s, 0, 8);
    s.x = x;
    s.y = y;
    f(&s);
}
int test2(int x, int y) {
    struct S s;
    long long dummy = (unsigned char)x;
    __builtin_memcpy(&s, &dummy, 8);
    s.x = x;
    s.y = y;
    f(&s);
}
```

Whether one of them is better might depend on the context... for test2, the zero-extend can be folded away in some cases.

I'd prefer to handle this on the LLVM side if possible (memcpyopt, or something like that); it's probably tricky to catch all the relevant cases in clang.

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