Auto merge of #119099 - fmease:always-const-trait-bounds, r=fee1-dead

 Introduce `const Trait` (always-const trait bounds)

Feature `const_trait_impl` currently lacks a way to express “always const” trait bounds. This makes it impossible to define generic items like fns or structs which contain types that depend on const method calls (\*). While the final design and esp. the syntax of effects / keyword generics isn't set in stone, some version of “always const” trait bounds will very likely form a part of it. Further, their implementation is trivial thanks to the `effects` backbone.

Not sure if this needs t-lang sign-off though.

(\*):

```rs
#![feature(const_trait_impl, effects, generic_const_exprs)]

fn compute<T: const Trait>() -> Type<{ T::generate() }> { /*…*/ }

struct Store<T: const Trait>
where
    Type<{ T::generate() }>:,
{
    field: Type<{ T::generate() }>,
}
```

Lastly, “always const” trait bounds are a perfect fit for `generic_const_items`.

```rs
#![feature(const_trait_impl, effects, generic_const_items)]

const DEFAULT<T: const Default>: T = T::default();
```

Previously, we (oli, fee1-dead and I) wanted to reinterpret `~const Trait` as `const Trait` in generic const items which would've been quite surprising and not very generalizable.
Supersedes #117530.

---

cc `@oli-obk`

As discussed
r? fee1-dead (or compiler)

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