OpenMP/Fortran: Fix handling of optional is_device_ptr + bind(C) [PR108546]

For is_device_ptr, optional checks should only be done before calling
libgomp, afterwards they are NULL either because of absent or, by
chance, because it is unallocated or unassociated (for pointers/allocatables).

Additionally, it fixes an issue with explicit mapping for 'type(c_ptr)'.

	PR middle-end/108546

gcc/fortran/ChangeLog:

	* trans-openmp.cc (gfc_trans_omp_clauses): Fix mapping of
	type(C_ptr) variables.

gcc/ChangeLog:

	* omp-low.cc (lower_omp_target): Remove optional handling
	on the receiver side, i.e. inside target (data), for
	use_device_ptr.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/is_device_ptr-3.f90: New test.
	* testsuite/libgomp.fortran/use_device_ptr-optional-4.f90: New test.