[Syntax Breaking] Rename DefaultImpl to AutoImpl

DefaultImpl is a highly confusing name for what we now call auto impls,
as in `impl Send for ..`. The name auto impl is not formally decided
but for sanity anything is better than `DefaultImpl` which refers
neither to `default impl` nor to `impl Default`.

src/librustc_passes/ast_validation.rs

@@ -213,7 +213,7 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                                         item.span,
                                         Some("place qualifiers on individual impl items instead"));
             }
-            ItemKind::DefaultImpl(..) => {
+            ItemKind::AutoImpl(..) => {
                 self.invalid_visibility(&item.vis, item.span, None);
             }
             ItemKind::ForeignMod(..) => {