Translate allocate directive (OpenMP 5.0).

Translate allocate directive (OpenMP 5.0).

Backport of a patch posted at
https://gcc.gnu.org/pipermail/gcc-patches/2022-January/588369.html

gcc/fortran/ChangeLog:

	* trans-openmp.c (gfc_trans_omp_clauses): Handle OMP_LIST_ALLOCATOR.
	(gfc_trans_omp_allocate): New function.
	(gfc_trans_omp_directive): Handle EXEC_OMP_ALLOCATE.

gcc/ChangeLog:

	* tree-pretty-print.c (dump_omp_clause): Handle OMP_CLAUSE_ALLOCATOR.
	(dump_generic_node): Handle OMP_ALLOCATE.
	* tree.def (OMP_ALLOCATE): New.
	* tree.h (OMP_ALLOCATE_CLAUSES): Likewise.
	(OMP_ALLOCATE_DECL): Likewise.
	(OMP_ALLOCATE_ALLOCATOR): Likewise.
	* tree.c (omp_clause_num_ops): Add entry for OMP_CLAUSE_ALLOCATOR.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/allocate-6.f90: New test.