Fix handling of const params defaults that ref Self & generalize diag

We used to lower such bad defaulted const args in trait object types to
`{type error}`; now correctly lower them to `{const error}`.

The added tests used to ICE prior to this change.

tests/crashes/136063.rs

@@ -1,6 +0,0 @@
-//@ known-bug: #136063
-#![feature(generic_const_exprs)]
-trait A<const B: u8 = X> {}
-impl A<1> for bool {}
-fn bar(arg : &dyn A<x>) { bar(true) }
-pub fn main() {}

tests/crashes/137260.rs

@@ -1,11 +0,0 @@
-//@ known-bug: #137260
-#![feature(generic_const_exprs)]
-#![allow(incomplete_features)]
-
-trait Iter<const N: usize = { 1 + true }> {}
-
-fn needs_iter<const N: usize, T: Iter<N>>() {}
-
-fn test() {
-    needs_iter::<1, dyn Iter<()>>();
-}

tests/crashes/137514.rs

@@ -1,9 +0,0 @@
-//@ known-bug: #137514
-//@ needs-rustc-debug-assertions
-#![feature(generic_const_exprs)]
-
-trait Bar<const N: usize> {}
-
-trait BB = Bar<{ 1i32 + 1 }>;
-
-fn foo(x: &dyn BB) {}