[Clang] Type-punning through an uninitialized structure member

I found the С code that looked something like this:
```c
union U{
int i;
struct {int i; int j;} s;
};
union U u;
u.i = 42; // U::i is active member;
u.s.j = 42; // U::s is the active member with initialized s.j, s.i is the indeterminate value;
int i = u.s.i; // UB? accessing a value while it is indeterminate
int i2 = u.i; // UB? U::i does not have the expected value, even if type-punning is supported
```
In this case, it's difficult for me to even assess whether type-punching is used here (at the moment, I think it's not)?

related:
https://gitlab.winehq.org/wine/wine/-/merge_requests/10547

> C-standart (ISO/IEC 9899:TC2 Committee Draft — May 6, 2005 WG14/N1124):

>
> J.2 Undefined behavior
> — The value of an object with automatic storage duration is used while it is indeterminate (6.2.4, 6.7.8, 6.8).

> 6.7.8 Initialization
> 10 If an object that has automatic storage duration is not initialized explicitly, its value is indeterminate.

Start with the union example in the issue and read the cited ISO/IEC 9899:TC2 sections 6.2.4, 6.7.8, 6.8, and J.2. Compare the interpretation with the linked Wine merge request. Done means establishing and documenting whether the accesses have undefined behavior and whether the issue represents a Clang problem.

Written by the indexing model from the issue text.