[Flang] Update the fix of PR 80738 to cover generic interface inside modules (#81087)

The following test cases crashes. The problem is that the fix for PR
https://github.com/llvm/llvm-project/pull/80738 is not quite complete.
It should `GetUltimate()` of the `interface_` before check if it is
generic.


```
  MODULE M

    CONTAINS

    FUNCTION Int(Arg)
    INTEGER :: Int, Arg
      Int = Arg
    END FUNCTION

    FUNCTION Int8(Arg)
    INTEGER(8) :: Int8, Arg
      Int8 = 8_8
    END FUNCTION

  END MODULE

  MODULE M1
  USE M

    INTERFACE Int8
      MODULE PROCEDURE  Int
      MODULE PROCEDURE  Int8
    END INTERFACE

  END MODULE

  PROGRAM PtrAssignGen
  USE M
  USE M1
  IMPLICIT NONE

  INTERFACE Int
    MODULE PROCEDURE  Int
    MODULE PROCEDURE  Int8
  END INTERFACE

  PROCEDURE(Int8),   POINTER :: PtrInt8

  PtrInt8 => Int8
  IF ( PtrInt8(100_8) .NE. 8_8 ) ERROR STOP 12

  END
  ```