OpenMP/Fortran: Fix loop-iter var privatization with !$OMP LOOP [PR108512]

OpenMP/Fortran: Fix loop-iter var privatization with !$OMP LOOP [PR108512]

For 'parallel', loop-iteration variables are marked are marked as 'private',
unless they either appear in an omp do/simd loop or an data-sharing clause
already exists for those on 'parallel'. 'omp loop' wasn't handled, leading
to (potentially) multiple data-sharing clauses in gfc_resolve_do_iterator
as omp_current_ctx pointed to the 'parallel' directive, ignoring the
in-betwen 'loop' directive.

The latter lead to a bogus diagnostic - or rather an ICE as the source
location var contained only '\0'.

Additionally, several 'case EXEC_OMP...LOOP' have been added to call the
right resolution function and likewise for '{masked,master} taskloop'.

gcc/fortran/ChangeLog:

	PR fortran/108512
	* openmp.cc (gfc_resolve_omp_parallel_blocks): Handle combined 'loop'
	directives.
	(gfc_resolve_do_iterator): Set a source location for added
	'private'-clause arguments.
	* resolve.cc (gfc_resolve_code): Call gfc_resolve_omp_do_blocks
	also for EXEC_OMP_LOOP and gfc_resolve_omp_parallel_blocks for
	combined directives with loop + '{masked,master} taskloop (simd)'.

gcc/testsuite/ChangeLog:

	PR fortran/108512
	* gfortran.dg/gomp/loop-5.f90: New test.
	* gfortran.dg/gomp/loop-2.f90: Update dg-error.
	* gfortran.dg/gomp/taskloop-2.f90: Update dg-error.