Spurious warning in C++11 when attempting to define a constexpr destructor

In C++11 we are not allowed to declare/define a `constexpr` destructor, we obtain the proper diagnostic for this in clang but if we attempt to also define is then clang provides a completely spurious diagnostic as well: https://godbolt.org/z/Geq7Th7oh

```cpp
struct A {
constexpr ~A(){}
};
```

Diagnostic:

```console
:2:3: error: destructor cannot be declared constexpr
2 | constexpr ~A(){}
| ^
:2:13: error: constexpr function's return type 'void' is not a literal type
2 | constexpr ~A(){}
| ^
```

The second diagnostic is just not helpful at all.

Start by compiling the provided C++11 reproducer and compare Clang's diagnostics with the linked Godbolt example. Trace the semantic-analysis path that diagnoses constexpr destructors, then add or update a regression test for the snippet. Done means only the invalid-destructor diagnostic is emitted, without the spurious return-type diagnostic.

Written by the indexing model from the issue text.