Add tag_for_variant query

This query allows for sharing code between `rustc_const_eval` and
`rustc_transmutability`.

Also moves `DummyMachine` to `rustc_const_eval`.

compiler/rustc_const_eval/src/lib.rs

@@ -40,6 +40,7 @@ rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
 pub fn provide(providers: &mut Providers) {
     const_eval::provide(providers);
+    providers.tag_for_variant = const_eval::tag_for_variant_provider;
     providers.eval_to_const_value_raw = const_eval::eval_to_const_value_raw_provider;
     providers.eval_to_allocation_raw = const_eval::eval_to_allocation_raw_provider;
     providers.eval_static_initializer = const_eval::eval_static_initializer_provider;