Rollup merge of #87770 - BoxyUwU:cec-drop-impl, r=lcnr

permit drop impls with generic constants in where clauses

Fixes #79248

`==` is not sufficient to check for equality between unevaluated consts which causes the above issue because the const in `[(); N - 1]:` on the impl and the const in `[(); N - 1]:` on the struct def are not seen as equal. Any predicate that can contain an unevaluated const cant use `==` here as it will cause us to incorrectly emit an error.

I dont know much about chalk but it seems like we ought to be relating the `TypeWellFormedFromEnv` instead of `==` as it contains a `Ty` so I added that too...

r? ``````@lcnr``````

src/test/ui/const-generics/const_evaluatable_checked/drop_impl.rs

@@ -0,0 +1,16 @@
+//check-pass
+#![feature(const_generics, const_evaluatable_checked)]
+#![allow(incomplete_features)]
+
+struct Foo<const N: usize>
+where
+    [(); N + 1]: ;
+
+impl<const N: usize> Drop for Foo<N>
+where
+    [(); N + 1]: ,
+{
+    fn drop(&mut self) {}
+}
+
+fn main() {}