Improve errors on const mismatches

[b-naber]

Fixes #86198

This allows us to suggest a correct anonymous constant in case there's a mismatch, e.g. in:

trait Trait {}
struct HasCastInTraitImpl<const N: usize, const M: usize>;
impl<const M: usize> Trait for HasCastInTraitImpl<M, { M + 1 }> {}
pub struct HasTrait<T: Trait>(T);
fn foo<const N: usize>() -> HasTrait<HasCastInTraitImpl<{ N + 1 }, { N + 1}>> { loop {} }

we now get a mismatch const error of the following form:

error[E0308]: mismatched types
  --> $DIR/mismatched-const-errors.rs:15:30
   |
LL | fn foo2<const N: usize>() -> HasTrait<HasCastInTraitImpl<{ N + 1}, { N + 1 }>> {
   |                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `{ N + 1 }`, found `{ N + 1 + 1 }`
   |
   = note: expected type `{ N + 1 }`
              found type `{ N + 1 + 1 }`

instead of the found { M + 1 } constant that was previously output.