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_traits/src/chalk/db.rs

@@ -739,7 +739,7 @@ fn bound_vars_for_item(tcx: TyCtxt<'tcx>, def_id: DefId) -> SubstsRef<'tcx> {
             tcx.mk_region(ty::RegionKind::ReLateBound(ty::INNERMOST, br)).into()
         }
 
-        ty::GenericParamDefKind::Const => tcx
+        ty::GenericParamDefKind::Const { .. } => tcx
             .mk_const(ty::Const {
                 val: ty::ConstKind::Bound(ty::INNERMOST, ty::BoundVar::from(param.index)),
                 ty: tcx.type_of(param.def_id),