Rename impl_constness to constness

The current code is a basis for `is_const_fn_raw`, and `impl_constness`
is no longer a valid name, which is previously used for determining the
constness of impls, and not items in general.

compiler/rustc_middle/src/ty/mod.rs

@@ -2309,7 +2309,7 @@ impl<'tcx> TyCtxt<'tcx> {
     #[inline]
     pub fn is_const_fn_raw(self, def_id: DefId) -> bool {
         matches!(self.def_kind(def_id), DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(..))
-            && self.impl_constness(def_id) == hir::Constness::Const
+            && self.constness(def_id) == hir::Constness::Const
     }
 
     #[inline]