"In light of the above, the Board finds that, in the August 20, 2009, rating decision granting service connection for schizophrenia, bipolar disorder aggravated by PTSD, the provisions of 38 C.F.R. § 3.156(c) were not correctly applied in determining the appropriate effective date; that the error was undebatable; and that, but for the error, an effective date of July 23, 1992—the day following the Veteran’s separation from service—would have been awarded for the grant of his September 1992 claim