Rollup merge of #122784 - jswrenn:tag_for_variant, r=compiler-errors

Add `tag_for_variant` query

This query allows for sharing code between `rustc_const_eval` and `rustc_transmutability`. It's a precursor to a PR I'm working on to entirely replace the bespoke layout computations in `rustc_transmutability`.

r? `@compiler-errors`

compiler/rustc_mir_transform/src/known_panics_lint.rs

@@ -6,6 +6,7 @@
 
 use std::fmt::Debug;
 
+use rustc_const_eval::const_eval::DummyMachine;
 use rustc_const_eval::interpret::{
     format_interp_error, ImmTy, InterpCx, InterpResult, Projectable, Scalar,
 };
@@ -20,7 +21,6 @@ use rustc_middle::ty::{self, ConstInt, ParamEnv, ScalarInt, Ty, TyCtxt, TypeVisi
 use rustc_span::Span;
 use rustc_target::abi::{Abi, FieldIdx, HasDataLayout, Size, TargetDataLayout, VariantIdx};
 
-use crate::dataflow_const_prop::DummyMachine;
 use crate::errors::{AssertLint, AssertLintKind};
 use crate::MirLint;