-Wunused-variable incorrectly triggering on self-init

Consider the following example:

```c
struct [[gnu::warn_unused]] S {
};

void func() {
struct S s1 = s1;
}
```

Language | Clang | GCC
---| --- | ---
C | No diagnostic | No diagnostic
C++ | Diagnostic | No diagnostic

https://godbolt.org/z/nafGTonor

I believe Clang's diagnostic in C++ is a false positive; mentioning the variable in its own initialization should silence the diagnostic because the variable is being used.

Start by reproducing the self-initialization example from the issue, using the linked Godbolt comparison to check Clang and GCC in both C and C++. Trace the -Wunused-variable diagnostic for the C++ case and identify the relevant compiler tests or diagnostic path. Done means the self-reference no longer produces a false positive while the warning remains correct for genuinely unused variables.

Written by the indexing model from the issue text.