This change affects only mode with IR f/o generator
The old way of overridden computation with fir2ir f/o generator relied
on the fact that fir2ir generator creates IR for all f/o and fills
its caches. But with IR f/o generator enabled, we don't call fir2ir
generator, so some caches are missing. And for this mode it's enough
to acquire the symbol using the original declaration symbol and the
lookup tag of the corresponding supertype
Relates to KT-64202
Dmitriy Novozhilov