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_middle/src/query/mod.rs

@@ -1042,6 +1042,13 @@ rustc_queries! {
         }
     }
 
+    /// Computes the tag (if any) for a given type and variant.
+    query tag_for_variant(
+        key: (Ty<'tcx>, abi::VariantIdx)
+    ) -> Option<ty::ScalarInt> {
+        desc { "computing variant tag for enum" }
+    }
+
     /// Evaluates a constant and returns the computed allocation.
     ///
     /// **Do not use this** directly, use the `eval_to_const_value` or `eval_to_valtree` instead.