Auto merge of #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: <321849610>

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -378,7 +378,7 @@ impl<'ll, 'tcx> IntrinsicCallMethods<'tcx> for Builder<'_, 'll, 'tcx> {
                 }
             }
 
-            _ => bug!("unknown intrinsic '{}'", name),
+            _ => bug!("unknown intrinsic '{}' -- should it have been lowered earlier?", name),
         };
 
         if !fn_abi.ret.is_ignore() {