Check assoc consts and tys later like assoc fns

[mu001999]

This PR

1. checks assoc consts and tys later like assoc fns
2. marks assoc consts appear in poly-trait-ref live

For assoc consts, considering

#![deny(dead_code)]

trait Tr { // ERROR trait `Tr` is never used
    const I: Self;
}

struct Foo; //~ ERROR struct `Foo` is never constructed

impl Tr for Foo {
    const I: Self = Foo;
}

fn main() {}

Current this will produce unused I instead of unused Tr and Foo (play), because const I: Self = Foo; will be added into the worklist at first:

error: associated constant `I` is never used
 --> src/main.rs:4:11
  |
3 | trait Tr { // ERROR trait `Tr` is never used
  |       -- associated constant in this trait
4 |     const I: Self;
  |           ^
  |
note: the lint level is defined here
 --> src/main.rs:1:9
  |
1 | #![deny(dead_code)]
  |         ^^^^^^^^^

error: could not compile `playground` (bin "playground") due to 1 previous error

This also happens to assoc tys, see the new test

Fixes #126729

r? @petrochenkov