Static variable template with deduced type within a class template is rejected

Consider the following example:
```cpp
template
struct A {
template
static constexpr auto value = 5;
};

decltype(A::value) x{};
```
Clang has been rejecting this since 9 (https://godbolt.org/z/8xMYEW51a), but other compilers accept (https://godbolt.org/z/Wcnz4K8f6):
```
:13:37: error: illegal initializer type 'decltype(A::value)' (aka 'const auto')
13 | decltype(A::value) x{};
| ^
```

Consider the following example derived from the first one that adds a type alias:
```cpp
template
struct A {
template
static constexpr auto value = 5;

using result_t = decltype(value);
};

decltype(A::value) x{};
```
Clang has been rejecting this since 12 with the same diagnostic as above (https://godbolt.org/z/zqaExM1a8), but other compilers accept (https://godbolt.org/z/zxx1acje6). Since 21 we issue a different diagnostic:
```
:9:19: error: the type of variable template specialization 'value' declared with deduced type 'const auto' depends on itself
9 | decltype(A::value) x{};
| ^
```

Related: #147992, #134526, #45408

Start by reducing and running the two C++ reproducer examples, then read related issues #147992, #134526, and #45408 to understand the existing context. Compare Clang's diagnostics and behavior with the other compilers shown in the linked Compiler Explorer cases; done means the examples are handled consistently with the intended standard behavior and covered by an appropriate compiler test.

Written by the indexing model from the issue text.