Gimplify allocate directive (OpenMP 5.0).

Gimplify allocate directive (OpenMP 5.0).

Backport of a patch posted at
https://gcc.gnu.org/pipermail/gcc-patches/2022-January/588371.html

gcc/ChangeLog:

	* doc/gimple.texi: Describe GIMPLE_OMP_ALLOCATE.
	* gimple-pretty-print.cc (dump_gimple_omp_allocate): New function.
	(pp_gimple_stmt_1): Call it.
	* gimple.cc (gimple_build_omp_allocate): New function.
	* gimple.def (GIMPLE_OMP_ALLOCATE): New node.
	* gimple.h (enum gf_mask): Add GF_OMP_ALLOCATE_KIND_MASK,
	GF_OMP_ALLOCATE_KIND_ALLOCATE and GF_OMP_ALLOCATE_KIND_FREE.
	(struct gomp_allocate): New.
	(is_a_helper <gomp_allocate *>::test): New.
	(is_a_helper <const gomp_allocate *>::test): New.
	(gimple_build_omp_allocate): Declare.
	(gimple_omp_subcode): Replace GIMPLE_OMP_TEAMS with
	GIMPLE_OMP_ALLOCATE.
	(gimple_omp_allocate_set_clauses): New.
	(gimple_omp_allocate_set_kind): Likewise.
	(gimple_omp_allocate_clauses): Likewise.
	(gimple_omp_allocate_kind): Likewise.
	(CASE_GIMPLE_OMP): Add GIMPLE_OMP_ALLOCATE.
	* gimplify.cc (gimplify_omp_allocate): New.
	(gimplify_expr): Call it.
	* gsstruct.def (GSS_OMP_ALLOCATE): Define.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/allocate-6.f90: Add tests.