[flang] Make C_F_POINTER a generic interface

[klausler]

Summary

The intrinsic procedure C_F_POINTER needs to be a generic interface
in intrinsic module ISO_C_BINDING. (It also needs to be implemented,
but that remains a TODO for either lowering or the runtime.)

flang/module/iso_c_binding.f90

#endif
   character(kind=c_char, len=1), parameter :: c_vertical_tab =  achar(11)
 
   interface c_associated
     module procedure c_associated_c_ptr
     module procedure c_associated_c_funptr
   end interface
   private :: c_associated_c_ptr, c_associated_c_funptr
 
+  interface c_f_procpointer
+    module procedure c_f_procpointer
+  end interface
+
   ! gfortran extensions
   integer, parameter :: &
     c_float128 = 16, &
     c_float128_complex = c_float128
 
  contains
 
   pure logical function c_associated_c_ptr(c_ptr_1, c_ptr_2)
contains
   end function c_associated_c_funptr
 
   function c_funloc(x)
     type(c_funptr) :: c_funloc
     external :: x
     c_funloc = c_funptr(loc(x))
   end function c_funloc
 
-  ! TODO c_f_procpointer
+  subroutine c_f_procpointer(cptr, fptr)
+    type(c_funptr), intent(in) :: cptr
+    procedure(), pointer, intent(out) :: fptr
+    ! TODO: implement
+  end subroutine c_f_procpointer
 
 end module iso_c_binding