`-Wunused` false positive on constexpr function used to compute return type

The following produces a warning ([godbolt](https://godbolt.org/z/crKW95456)):

```
#include

static constexpr bool returnInt(int) { return true; }

template
std::conditional_t make() {
return T{};
}

int makeInt() { return make(); }
float makeFloat() { return make(); }
```

Warning:
```
function 'returnInt' is not needed and will not be emitted
```

Start with the linked Godbolt reproducer and confirm the -Wunused warning for the constexpr returnInt function used in the conditional return type. Trace the compiler's unused-function diagnostic for this case; done means the valid function no longer produces a false-positive warning while genuine unused functions still do.

Written by the indexing model from the issue text.