False positive `-Wunneeded-internal-declaration` for function only called in discarded statement

https://godbolt.org/z/5j7EYsYTh
```cpp
static void f() {}

int main() {
if constexpr (false) { f(); }
}
```
```
:1:13: warning: function 'f' is not needed and will not be emitted [-Wunneeded-internal-declaration]
1 | static void f() {}
| ^
1 warning generated.
```
This feels like a false positive because normally, uses in discarded statements are still counted for the purpose of diagnostics. For example, I recall that older versions of Clang warned `-Wunused-parameter` historically, but no longer do for the following code:
```cpp
int g(int x) {
if constexpr (false) {
return x;
} else {
return 0;
}
}
```

The code is artificial; I ran into this issue in practice with:
```cpp
if constexpr (is_debug_build)
```
(because preprocessing is lame, and I didn't want `#if`)

Start by reproducing the linked Godbolt example with Clang and inspect the handling of -Wunneeded-internal-declaration for a function referenced only inside a discarded if constexpr statement. Compare it with the discarded-statement behavior described for -Wunused-parameter; done means the artificial example no longer reports a false-positive warning while genuinely unused functions remain diagnosed.

Written by the indexing model from the issue text.