let_unit_with_type_underscore: make early-pass

[ada4a]

This kind of supersedes #15386 -- by making the lint early-pass, we get access to TyKind::Parens that surround the type ascription. And with that, we can return to the simpler calculation of span_to_remove.

The biggest hurdle was is_from_proc_macro -- calling that function required me to impl WithSearchPat for rustc_ast::Ty, i.e. ast_ty_search_pat, which I based on ty_search_pat. Since that's a larger change, I could extract it into a PR of its own.

changelog: none

[rustbot]

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[samueltardieu]

I won't be able to properly review this PR in the next two weeks, I'll reroll the dice.
r? clippy