(Mormon group formed as religious trust to preserve and advance religious doctrines of Mormonism as espoused by founder Joseph Smith, Jr., sought tax-exempt status under I.R.C. Sec. 501(a) as an organization described in I.R.C. Sec. 501(d); IRS denied exemption on basis that group's belief in and promotion of polygamy via "Celestial Marriage" was illegal under state and federal law and a charitable trust cannot be created for an illegal purpose).