interpret: remove LocalValue::Unallocated, add Operand::Uninit

Operand::Uninit is an *allocated* operand that is fully uninitialized.
This lets us lazily allocate the actual backing store of *all* locals (no matter their ABI).

I also reordered things in pop_stack_frame at the same time.
I should probably have made that a separate commit...

compiler/rustc_const_eval/src/const_eval/eval_queries.rs

@@ -189,6 +189,7 @@ pub(super) fn op_to_const<'tcx>(
                 let len: usize = len.try_into().unwrap();
                 ConstValue::Slice { data, start, end: start + len }
             }
+            Immediate::Uninit => to_const_value(&op.assert_mem_place()),
         },
     }
 }