[mypyc] Fix is_native_ref_expr for class attrs

[jhance]

This addresses mypyc/mypyc#932 in which
in some cases before we were incorrectly not determining that the symbol was native.

Because the problematic implementation of is_native_ref_expr is present in multiple places,
the one in builder.py is rewritten to use the one in mapper.py.

Additional run tests are added to cover the case. Use of a dictionary here prevents falling
under a secondary, more optimized codepath, that avoided the issue entirely.

[JukkaL]

Looks good!