Improve dead code analysis

[mu001999]

Fixes #120770

1. check impl items later if self ty is private although the trait method is public, cause we must use the ty firstly if it's private
2. mark the adt live if it appears in pattern, like generic argument, this implies the use of the adt
3. based on the above, we can handle the case that private adts impl Default, so that we don't need adding rustc_trivial_field_reads on Default, and the logic in should_ignore_item

r? @pnkfelix

[rustbot]

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

[pnkfelix]

@bors r+

[bors]

📌 Commit 0adb825 has been approved by pnkfelix

It is now in the queue for this repository.