fix: Filter suggestion parts that match existing code

[Muscraft]

While testing my changes to make rustc use annotate-snippets, I encountered a new clippy test failure stemming from two suggestion output changes in #145273. The new output in these two cases feels like a regression as it is not as clear as the old output, and adds unnecessary information.

Before #145273 (Diff style)

After #145273 ("multi-line" style)

The reason for the change was that a new suggestion part (which matches existing code) was added on a different line than the existing parts, causing the suggestion style to change from Diff to "multi-line". Since this new part matches existing code, no code changes show up in the output for it, but it still makes the suggestion style "multi-line" when it doesn't need to be.

To get the old output back, I made it so that suggestion parts that perfectly match existing code get filtered out.

[rustbot]

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[rustbot]

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[petrochenkov]

@bors r+

[bors]

📌 Commit f696cd8 has been approved by petrochenkov

It is now in the queue for this repository.