Auto merge of #86580 - BoxyUwU:cgd-subst-ice, r=nikomatsakis

dont provide fwd declared params to cg defaults

Fixes #83938

```rust
#![feature(const_evaluatable_checked, const_generics, const_generics_defaults)]
#![allow(incomplete_features)]

pub struct Bar<const N: usize, const M: usize = { N + 1 }>;
pub fn foo<const N1: usize>() -> Bar<N1> { loop {} }

fn main() {}
```
This PR makes this code no longer ICE, it was ICE'ing previously because when building substs for `Bar<N1>` we would subst the anon ct: `ConstKind::Unevaluated({N + 1}, substs: [N, M])` with substs of `[N1]`. the anon const has forward declared params supplied though so we end up trying to substitute the provided `M` param which causes the ICE.

This PR doesn't handle the predicates of the const so
```rust
trait Foo<const N: usize> { const Assoc: usize; }
pub struct Bar<const N: usize = { <()>::Assoc }> where (): Foo<N>;
```
Resolves to `<() as Foo<N>>::Assoc` which can allow for using fwd declared params indirectly.

```rust
trait Foo<const N: usize> {}
struct Bar<const N: usize = { 2 + 3 }> where (): Foo<N>;
```
This code also ICEs under this PR because instantiating the default's predicates causes an ICE as predicates_of contains predicates with fwd declared params

PR was briefly discussed [in this zulip thread](https://rust-lang.zulipchat.com/#narrow/stream/260443-project-const-generics/topic/evil.20preds.20in.20param.20env.20.2386580)

compiler/rustc_hir/src/hir.rs

@@ -1422,6 +1422,9 @@ pub type Lit = Spanned<LitKind>;
 /// These are usually found nested inside types (e.g., array lengths)
 /// or expressions (e.g., repeat counts), and also used to define
 /// explicit discriminant values for enum variants.
+///
+/// You can check if this anon const is a default in a const param
+/// `const N: usize = { ... }` with `tcx.hir().opt_const_param_default_param_hir_id(..)`
 #[derive(Copy, Clone, PartialEq, Eq, Encodable, Debug, HashStable_Generic)]
 pub struct AnonConst {
     pub hir_id: HirId,