d4605f5816
This is needed to get rid of the diamond hierarchy:
IrMemberAccessExpression
/ \
/ \
IrFunctionAccessExpression IrCallableReference
\ /
\ /
IrFunctionReference
In the subsequent commit, IrFunctionReference no longer inherits from
IrFunctionAccessExpression; the more precise type of `val symbol:
IrFunctionSymbol` is now carried via the generic type argument.
This will help to refactor IR element hierarchy from interfaces to
classes, improving performance of visitors and transformers.