fix false negative for unnecessary_unwrap

[relaxcn]

changelog: Fix [unnecessary_unwrap] false negative when any assignment occurs in if branch (regardless of any variable).

Fixes: #14725

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[samueltardieu]

LGTM

[samueltardieu]

Incidentally, your PR caused #14761 and #14763 (found during review). Thanks!

[relaxcn]

Thank you, I didn't notice them because I'm new.

I think if we want to build a mature tool, we must build some top-level interfaces to define lints so that the lint developers could ignore the rustc internal APIs changes.

In another words, I mean 'The Engine'.

If the lint could be written with a human-friendly way(top-level APIs or another descriptive language) provided by 'The Engine', we don't need to modify every lint when the rustc API changes.

In the long run, the benefits of building such an engine far outweigh the benefits of just creating clippy_utils.