[TySan] Type aliasing violations not detected on returned struct.

Reproducer (C):
```c
struct S {
int a;
float b;
};

struct S returns_s(void) {
struct S x;
x.a = 1;
x.b = 1.0f;
return x;
}

int main(void) {
struct S x = returns_s();

// This should produce an aliasing violation.
float aliased = *((float *) &x.a);
}
```

When compiled with `clang -fsanitize=type`, the sanitizer does not catch the type aliasing violation reading `x.a` through a float pointer. If `x` has its members assigned in main, then the aliasing violation is correctly reported.

Start by compiling the provided C reproducer with clang -fsanitize=type and compare it with the case where the struct members are assigned in main. Trace why the returned struct does not trigger the aliasing violation; done means the returned-struct case reports the violation without regressing the existing case.

Written by the indexing model from the issue text.