catch function argument evaluation order bugs with -Wsometimes-uninitialized

Consider something like:

```
int init(int* p);
void g(int a, int b);

void f(void) {
int x;
g(init(&x), x);
}
```

Assuming that `init` initializes `*p`, `x` may or may not be initialized in the call to `g` depending on the order in which the function call arguments are evaluated, which is unspecified.

Catching order of evaluation bugs in general is tricky, but in cases like this, it seems `-Wsometimes-uninitialized` could warn.

Start by reproducing the C example with the compiler's existing -Wsometimes-uninitialized warning behavior. Investigate the warning's handling of function-call argument evaluation, then add coverage showing that the order-dependent use of x is diagnosed without changing unrelated cases.

Written by the indexing model from the issue text.