split up #[rustc_deny_explicit_impl] attribute

This commit splits the `#[rustc_deny_explicit_impl(implement_via_object = ...)]` attribute
into two attributes `#[rustc_deny_explicit_impl]` and `#[rustc_do_not_implement_via_object]`.

This allows us to have special traits that can have user-defined impls but do not have the
automatic trait impl for trait objects (`impl Trait for dyn Trait`).

compiler/rustc_feature/src/builtin_attrs.rs

@@ -912,11 +912,20 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     rustc_attr!(
         rustc_deny_explicit_impl,
         AttributeType::Normal,
-        template!(List: "implement_via_object = (true|false)"),
+        template!(Word),
         ErrorFollowing,
         EncodeCrossCrate::No,
         "#[rustc_deny_explicit_impl] enforces that a trait can have no user-provided impls"
     ),
+    rustc_attr!(
+        rustc_do_not_implement_via_object,
+        AttributeType::Normal,
+        template!(Word),
+        ErrorFollowing,
+        EncodeCrossCrate::No,
+        "#[rustc_do_not_implement_via_object] opts out of the automatic trait impl for trait objects \
+        (`impl Trait for dyn Trait`)"
+    ),
     rustc_attr!(
         rustc_has_incoherent_inherent_impls, AttributeType::Normal, template!(Word),
         ErrorFollowing, EncodeCrossCrate::Yes,