Fix ReferenceStorage(T) atomic if T has no inner pointers (#14845)

It turns out the fix in #14730 made all `ReferenceStorage` objects non-atomic; `Crystal::ReferenceStorageType#reference_type` returns a reference type, whose `#has_inner_pointers?` always returns true since the reference itself is a pointer.
This PR fixes that again by adding a special case for `ReferenceStorage`.

src/compiler/crystal/codegen/types.cr

@@ -70,7 +70,7 @@ module Crystal
       when NamedTupleInstanceType
         self.entries.any? &.type.has_inner_pointers?
       when ReferenceStorageType
-        self.reference_type.has_inner_pointers?
+        self.reference_type.all_instance_vars.each_value.any? &.type.has_inner_pointers?
       when PrimitiveType
         false
       when EnumType