[Clang] Implementation divergence on uninitialized union

```c++
struct S {
S() {}
};

union U {
constexpr U() {}
constexpr ~U() {}

S s;
};

constinit U u;
```
is accepted by Clang, but rejected by GCC and MSVC. Not sure who is correct here.

Start by checking the C++ rules for constinit, constexpr union constructors and uninitialized union members, then compare the diagnostic and acceptance behavior across Clang, GCC and MSVC using the reproducer in the issue. No source file or test location is named; done means establishing which behavior is standard-conforming and identifying the compiler change or standards clarification needed.

Written by the indexing model from the issue text.