Add has_default to GenericParamDefKind::Const

This currently creates a field which is always false on GenericParamDefKind for future use when
consts are permitted to have defaults

Update const_generics:default locations

Previously just ignored them, now actually do something about them.

Fix using type check instead of value

Add parsing

This adds all the necessary changes to lower const-generics defaults from parsing.

Change P<Expr> to AnonConst

This matches the arguments passed to instantiations of const generics, and makes it specific to
just anonymous constants.

Attempt to fix lowering bugs

clippy_utils/src/ast_utils.rs

@@ -408,6 +408,10 @@ pub fn eq_use_tree(l: &UseTree, r: &UseTree) -> bool {
     eq_path(&l.prefix, &r.prefix) && eq_use_tree_kind(&l.kind, &r.kind)
 }
 
+pub fn eq_anon_const(l: &AnonConst, r: &AnonConst) -> bool {
+  eq_expr(&l.value, &r.value)
+}
+
 pub fn eq_use_tree_kind(l: &UseTreeKind, r: &UseTreeKind) -> bool {
     use UseTreeKind::*;
     match (l, r) {
@@ -418,10 +422,6 @@ pub fn eq_use_tree_kind(l: &UseTreeKind, r: &UseTreeKind) -> bool {
     }
 }
 
-pub fn eq_anon_const(l: &AnonConst, r: &AnonConst) -> bool {
-    eq_expr(&l.value, &r.value)
-}
-
 pub fn eq_defaultness(l: Defaultness, r: Defaultness) -> bool {
     matches!(
         (l, r),