The last time that Congress approved a sweeping overhaul of the federal tax code, in 1986, it created a tax credit meant to encourage the private sector to invest in affordable housing. It has grown into a $9 billion-a-year social program that has funded the construction of some three million apartments for low-income residents.But the Republican tax plan approved last month amounts to a vast cutback, making it much less likely that such construction will continue apace.