Auto merge of rust-lang#108442 - scottmcm:mir-transmute, r=oli-obk

Add `CastKind::Transmute` to MIR

~~Nothing actually produces it in this commit, so I don't know how to test it, but it also means it shouldn't be possible for it to break anything.~~

Includes lowering `transmute` calls to it, so it's used.

Zulip Conversation: <https://rust-lang.zulipchat.com/#narrow/stream/189540-t-compiler.2Fwg-mir-opt/topic/Good.20first.20isssue/near/321849610>

clippy_utils/src/qualify_min_const_fn.rs

@@ -176,6 +176,9 @@ fn check_rvalue<'tcx>(
             // FIXME(dyn-star)
             unimplemented!()
         },
+        Rvalue::Cast(CastKind::Transmute, _, _) => {
+            Err((span, "transmute can attempt to turn pointers into integers, so is unstable in const fn".into()))
+        },
         // binops are fine on integers
         Rvalue::BinaryOp(_, box (lhs, rhs)) | Rvalue::CheckedBinaryOp(_, box (lhs, rhs)) => {
             check_operand(tcx, lhs, span, body)?;