RS0046: Refactor call to 'LookupSymbols' to avoid name clash in arguments

Relates to #3352 

I could not find which `position` to pass to the `LookupSymbols` call to avoid a name clash between the current parameter and the next.

```csharp
public void M(string? sOpt, string? s) // How to ensure that the lookup for s returns the parameter?
{
}
```

See https://github.com/dotnet/roslyn-analyzers/pull/3352/files#diff-311b2f6fab1f2e00cefcbdf6e056dd96R202-R233