[Flang] Missing error for MODULE prefix in abstract interface body

Flang fails to diagnose an invalid `MODULE` prefix on a function declared inside an ABSTRACT INTERFACE block.

Expected behavior: The code should be rejected at compile time. Per the Fortran standard, MODULE may appear only in the function or subroutine of a module subprogram, or of a nonabstract interface body declared in the scoping unit of a module or submodule.

Actual behavior: Flang compiles the program successfully with no diagnostic.

Reproducer:

```Fortran
Module mod_test
Abstract Interface
Pure Integer Module Function f1(i)
Integer, Intent(In) :: i
End Function
End Interface
End Module

```