Deny more `~const` trait bounds
thereby fixing a family of ICEs (delayed bugs) for `feature(const_trait_impl, effects)` code.
As discussed
r? `@fee1-dead`
Previously, any associated function could have `~const` trait bounds on
generic parameters, which could lead to ICEs when these bounds were used
on associated functions of non-`#[const_trait] trait` or
non-`impl const` blocks.
Includes changes as per @fee1-dead's comments in #116210.