Permit ImmTy::_offset to equally-sized aggreate fields if they are uninhabited

Author: saethlin

compiler/rustc_const_eval/src/interpret/operand.rs

@@ -279,7 +279,7 @@ impl<'tcx, Prov: Provenance> ImmTy<'tcx, Prov> {
                     match (self.layout.abi, layout.abi) {
                         (Abi::Scalar(..), Abi::Scalar(..)) => true,
                         (Abi::ScalarPair(..), Abi::ScalarPair(..)) => true,
-                        _ => false,
+                        _ => layout.abi == Abi::Uninhabited,
                     },
                     "cannot project into {} immediate with equally-sized field {}\nouter ABI: {:#?}\nfield ABI: {:#?}",
                     self.layout.ty,