Handle cleanup of omp allocated variables (OpenMP 5.0).

Handle cleanup of omp allocated variables (OpenMP 5.0).

Currently we are only handling omp allocate directive that is associated
with an allocate statement.  This statement results in malloc and free calls.
The malloc calls are easy to get to as they are in the same block as allocate
directive.  But the free calls come in a separate cleanup block.  To help any
later passes finding them, an allocate directive is generated in the
cleanup block with kind=free. The normal allocate directive is given
kind=allocate.

Backport of a patch posted at
https://gcc.gnu.org/pipermail/gcc-patches/2022-January/588370.html

gcc/fortran/ChangeLog:

	* gfortran.h (struct access_ref): Declare new members
	omp_allocated and omp_allocated_end.
	* openmp.cc (gfc_match_omp_allocate): Set new_st.resolved_sym to
	NULL.
	(prepare_omp_allocated_var_list_for_cleanup): New function.
	(gfc_resolve_omp_allocate): Call it.
	* trans-decl.cc (gfc_trans_deferred_vars): Process omp_allocated.
	* trans-openmp.cc (gfc_trans_omp_allocate): Set kind for the stmt
	generated for allocate directive.

gcc/ChangeLog:

	* tree-core.h (struct tree_base): Add comments.
	* tree-pretty-print.cc (dump_generic_node): Handle allocate directive
	kind.
	* tree.h (OMP_ALLOCATE_KIND_ALLOCATE): New define.
	(OMP_ALLOCATE_KIND_FREE): Likewise.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/allocate-6.f90: Test kind of allocate directive.