Fix apparent bug in Layout::is_refcounted

It contradicts build_tag in compiler/gen_llvm/src/llvm/build.rs,
where we create a NonNullableUnwrapped by calling
reserve_with_refcount_union_as_block_of_memory

compiler/mono/src/layout.rs

@@ -968,14 +968,9 @@ impl<'a> Layout<'a> {
         use Layout::*;
 
         match self {
-            Union(variant) => {
-                use UnionLayout::*;
+            Union(UnionLayout::NonRecursive(_)) => false,
 
-                matches!(
-                    variant,
-                    Recursive(_) | NullableWrapped { .. } | NullableUnwrapped { .. }
-                )
-            }
+            Union(_) => true,
 
             RecursivePointer => true,