Do not warn for unused variables on trait method declaration

On the following code, gccrs should not warn about unused variables:

pub trait Foo {
    fn Bar(self) -> i32;
}

Since we are simply "declaring" (in C/C++ terms) a trait method, the self variable is not intended to be used here. However, each type that implement the Foo trait will need to use the self variable (or rename it) in their implementation of the Bar method.

The following code, which provides a default implementation for the trait method, should warn:

pub trait Foo {
    fn Bar(self) -> i32 {
        -1 // default implementation returns -1 for example
    }
}

So there should be a distinction between a default trait method implementation and just a 'declaration' like here for unused variables

Originally posted in https://github.com/Rust-GCC/gccrs/pull/540#discussion_r662380851

No source file or test is named. Start by tracing the unused-variable warning entry point for Rust trait methods, then compare declaration-only methods with default implementations. Done means declaration-only trait parameters do not warn, while parameters in default implementations still do.