interpret: rename base_pointer -> root_pointer

also in Miri, "base tag" -> "root tag"

compiler/rustc_const_eval/src/interpret/traits.rs

@@ -28,7 +28,7 @@ impl<'mir, 'tcx: 'mir, M: Machine<'mir, 'tcx>> InterpCx<'mir, 'tcx, M> {
         ensure_monomorphic_enough(*self.tcx, poly_trait_ref)?;
 
         let vtable_symbolic_allocation = self.tcx.reserve_and_set_vtable_alloc(ty, poly_trait_ref);
-        let vtable_ptr = self.global_base_pointer(Pointer::from(vtable_symbolic_allocation))?;
+        let vtable_ptr = self.global_root_pointer(Pointer::from(vtable_symbolic_allocation))?;
         Ok(vtable_ptr.into())
     }