[Flang] Support for NULL() and procedure in structure constructor for procedure pointer component. (#85991)

This PR fixes a subset of procedure pointer component initialization in
structure constructor.
It covers
  1. NULL()
  2. procedure
  
 For example:
 ```
   MODULE M
    TYPE :: DT
      !PROCEDURE(Fun), POINTER, NOPASS :: pp1
      PROCEDURE(Fun), POINTER :: pp1
    END TYPE

    CONTAINS

    INTEGER FUNCTION Fun(Arg)
     class(dt) :: arg
    END FUNCTION

  END MODULE

  PROGRAM MAIN
  USE M
  IMPLICIT NONE
  TYPE (DT), PARAMETER :: v1 = DT(NULL())
  TYPE (DT) :: v2
  v2 = DT(fun)
  END
  ```
  
Passing a procedure pointer itself or reference to a function that
returns a procedure pointer is TODO.