[clang] "incomplete type 'const A' is not a literal type" for a static constexpr data member

Not sure if this is a bug or not, bug gcc compiles this code just fine:

```c++
template
struct Mixin {
static constexpr Base base{};
};

struct A {
Mixin m;
};
```

```
:3:25: error: constexpr variable cannot have non-literal type 'const A'
3 | static constexpr Base base{};
| ^
:7:14: note: in instantiation of template class 'Mixin' requested here
7 | Mixin m;
| ^
:3:25: note: incomplete type 'const A' is not a literal type
3 | static constexpr Base base{};
| ^
:6:8: note: definition of 'A' is not complete until the closing '}'
6 | struct A {
| ^
```

https://godbolt.org/z/YbvW4Gq3n

Begin with the minimal C++ snippet and Godbolt link, comparing Clang and GCC behavior. Trace Clang's handling of the static constexpr member while A is incomplete and determine whether the diagnostic matches the language rules. Done means the issue is resolved with behavior or a diagnostic justified by the standard.

Written by the indexing model from the issue text.