Use generic Enum.IsDefined for typed source enum values

Replace resolved Enum.IsDefined(typeof(T), value) with IsDefined<T>(value) only for a nonnullable value attributed to the identical closed source enum owned by the current project and all effective targets .NET5+. Preserve the value expression, padding and call identity. Keep boxed, numeric, string, nullable and open values, unknown owners/imports, comments, deferred contexts and snapshots unchanged.