This change fixes FIR tests with JVM IR serialization enabled, some of
which start to fail after enabling IR fake override builder mode by
default.
In this case, even accessing the function body with `IrFunction.body`
does not work in some cases because it tries to load IR and ends up with
unbound symbols for some reason (most likely related to KT-63509). So we
avoid accessing the body by copying the `IrFunction.acceptChildren`
implementation and removing the `body.accept` call.
Using deep-copy to create a fake override is questionable here, but it
will need to be refactored separately.
Alexander Udalov
b88ce50500