Fortran: fix argument checking of intrinsics C_SIZEOF, C_F_POINTER [PR106500]

Fortran: fix argument checking of intrinsics C_SIZEOF, C_F_POINTER [PR106500]

The interpretation of the F2018 standard regarding valid arguments to the
intrinsic C_SIZEOF(X) was clarified in an edit to 18-007r1:

  https://j3-fortran.org/doc/year/22/22-101r1.txt

loosening restrictions and giving examples.  The F2023 text has:

! F2023:18.2.3.8  C_SIZEOF (X)
!
!   X shall be a data entity with interoperable type and type parameters,
!   and shall not be an assumed-size array, an assumed-rank array that
!   is associated with an assumed-size array, an unallocated allocatable
!   variable, or a pointer that is not associated.

where

! 3.41 data entity
!   data object, result of the evaluation of an expression, or the
!   result of the execution of a function reference

Update the checking code for interoperable arguments accordingly, and extend
to reject functions returning pointer as FPTR argument to C_F_POINTER.

gcc/fortran/ChangeLog:

	PR fortran/106500
	* check.cc (is_c_interoperable): Fix checks for C_SIZEOF.
	(gfc_check_c_f_pointer): Reject function returning a pointer as FPTR,
	and improve an error message.

gcc/testsuite/ChangeLog:

	PR fortran/106500
	* gfortran.dg/c_sizeof_6.f90: Remove wrong dg-error.
	* gfortran.dg/sizeof_2.f90: Adjust pattern.
	* gfortran.dg/c_f_pointer_tests_9.f90: New test.
	* gfortran.dg/c_sizeof_7.f90: New test.