Complete enum variants identically to structures.

In particular:
 - unit variants now display in the menu as "Variant", complete to "Variant", and display a detail of "Variant" (was "()")
 - tuple variants now display in the menu as "Variant(…)", complete to "Variant(${1:()})$0" (was "Variant($0)"), and display a detail of "Variant(type)" (was "(type)")
 - record variants now display in the menu as "Variant {…}", complete to "Variant { x: ${1:()} }$0" (was "Variant"), and display a detail of "Variant { x: type }" (was "{x: type}")

This behavior is identical to that of struct completions. In addition, tuple variants no longer set triggers_call_info, as to my understanding it's unnecessary now that we're emitting placeholders.

Tests have been updated to match, and the render::enum_variant::tests::inserts_parens_for_tuple_enums test has been removed entirely as it's covered by other tests (render::enum_detail_includes_{record, tuple}_fields, render::enum_detail_just_name_for_unit, render::pattern::enum_qualified).

crates/ide_completion/src/tests/use_tree.rs

@@ -167,7 +167,7 @@ impl Foo {
 }
 "#,
         expect![[r#"
-            ev Variant ()
+            ev Variant Variant
         "#]],
     );
 }