Rollup merge of rust-lang#127107 - mu001999-contrib:dead/enhance-2, r=pnkfelix

Improve dead code analysis

Fixes rust-lang#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``

Author: compiler-errors

core/src/default.rs

@@ -103,7 +103,6 @@ use crate::ascii::Char as AsciiChar;
 /// ```
 #[cfg_attr(not(test), rustc_diagnostic_item = "Default")]
 #[stable(feature = "rust1", since = "1.0.0")]
-#[cfg_attr(not(bootstrap), rustc_trivial_field_reads)]
 pub trait Default: Sized {
     /// Returns the "default value" for a type.
     ///