API Annotations: AnnAt disconnected for TYPEAPP

For the code

    type family F1 (a :: k) (f :: k -> Type) :: Type where
      F1 @Peano a f = T @Peano f a

the API annotation for the first @ is not attached to a SourceSpan in
the ParsedSource

Closes #16236

(cherry picked from commit cbfc9fca)