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

compiler/rustc_span/src/symbol.rs

@@ -384,6 +384,7 @@ symbols! {
         const_fn_fn_ptr_basics,
         const_fn_transmute,
         const_fn_union,
+        const_generic_defaults,
         const_generics,
         const_generics_defaults,
         const_if_match,