Extend implicit_clone to handle to_string calls

[smoelius]

Put another way, merge string_to_string into implicit_clone, as suggested here: #14173 (comment)

Note: I wrote this comment:

rust-clippy/clippy_lints/src/methods/implicit_clone.rs

Lines 43 to 45 in 6cdb7f6

	/// Note that `to_string` is not flagged by `implicit_clone`. So other lints that call
	/// `is_clone_like` and that do flag `to_string` must handle it separately. See, e.g.,
	/// `is_to_owned_like` in `unnecessary_to_owned.rs`.

Here is the context for why I wrote it: #7978 (comment)

Regardless, it's probably time for the comment to go away. Extending implicit_clone to handle to_string calls yields many hits within Clippy's codebase.

changelog: extend implicit_clone to handle to_string calls

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[smoelius]

@y21 Sorry for the force pushes. I think it should be good now.