Auto merge of rust-lang#16264 - roife:refactor/refactor-generate-delegate-trait, r=Veykril

internal: clean and enhance readability for `generate_delegate_trait`

Continue from rust-lang#16112

This PR primarily involves some cleanup and simple refactoring work, including:

- Adding numerous comments to layer the code and explain the behavior of each step.
- Renaming some variables to make them more sensible.
- Simplify certain operations using a more elegant approach.

The goal is to make this intricate implementation clearer and facilitate future maintenance.

In addition to this, the PR also removes redundant `path_transform` operations for `type_gen_args`.
Taking the example of `impl Trait<T1> for S<S1>`, where `S1` is considered. The struct `S` must be in the file where the user triggers code actions, so there's no need for the `path_transform`. Furthermore, before performing the transform, we've already renamed `S1`, ensuring it won't clash with existing generics parameters. Therefore, there's no need to transform it.

Author: bors