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_span/src/symbol.rs

@@ -1717,6 +1717,7 @@ symbols! {
         rustc_diagnostic_macros,
         rustc_dirty,
         rustc_do_not_const_check,
+        rustc_do_not_implement_via_object,
         rustc_doc_primitive,
         rustc_driver,
         rustc_dummy,