This is needed for two reasons:
1. Before this change companion object appeared in FirRegularClass
twice: in declarations list and in companionObject field. This may
trigger twice transform of it
2. It's very hard to implement generation of companion object by plugins
because if it is part of the tree then generated declaration must be
registered in FirProvider, which is inconsistent with other generated
declarations. Replacing FIR with symbol and removing custom logic of
visiting/transforming companion FIR allows us to just replace companionSymbol
in FirClass if plugin wants to generate it without any additional work
Dmitriy Novozhilov
75b6f7ca00