Various fixes for const_trait_impl
A few problems I found while making `Iterator` easier to const-implement.
1. More generous `~const Drop` check.
We check for nested fields with caller bounds.
For example, an ADT type with fields of types `A`, `B`, `C`, check if all of them are either:
- Bounded (`A: ~const Drop`, `B: Copy`)
- Known to be able to destruct at compile time (`C = i32`, `struct C(i32)`, `C = some_fn`)
2. Don't treat trait functions marked with `#[default_method_body_is_const]` as stable const fns when checking `const_for` and `const_try` feature gates.
I think anyone can review this, so no r? this time.
90dd7c03af