[clang] [clang-tools-extra] [clang][CodeComplete] skip explicit obj param in SignatureHelp (PR #146649)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

How does completion handle things like this?

```cpp
struct S {
    void f(this S, int);
};

int main() {
    (&S::f)(S{}, 0);
}
```

https://github.com/llvm/llvm-project/pull/146649