Refactor the two-phase check for impls and impl items
Refactor the two-phase dead code check to make the logic clearer and simpler:
1. adding assoc fn and impl into `unsolved_items` directly during the initial construction of the worklist
2. converge the logic of checking whether assoc fn and impl are used to `item_should_be_checked`, and the item is considered used only when its corresponding trait and Self adt are used
This PR only refactors as much as possible to avoid affecting the original functions. However, due to the adjustment of the order of checks, the test results are slightly different, but overall, there is no regression problem
Fixesrust-lang/rust#127911Fixesrust-lang/rust#128839
Extracted from https://github.com/rust-lang/rust/pull/128637.
r? petrochenkov
try-job: dist-aarch64-linux
896be667b8