Add parsing support for allocate directive (OpenMP 5.0)

Add parsing support for allocate directive (OpenMP 5.0)

Currently we only make use of this directive when it is associated
with an allocate statement.

gcc/fortran/ChangeLog:

	* dump-parse-tree.cc (show_omp_node): Handle EXEC_OMP_ALLOCATE.
	(show_code_node): Likewise.
	* gfortran.h (enum gfc_statement): Add ST_OMP_ALLOCATE.
	(OMP_LIST_ALLOCATOR): New enum value.
	(enum gfc_exec_op): Add EXEC_OMP_ALLOCATE.
	* match.h (gfc_match_omp_allocate): New function.
	* openmp.cc (enum omp_mask1): Add OMP_CLAUSE_ALLOCATOR.
	(OMP_ALLOCATE_CLAUSES): New define.
	(gfc_match_omp_allocate): New function.
	(resolve_omp_clauses): Add ALLOCATOR in clause_names.
	(omp_code_to_statement): Handle EXEC_OMP_ALLOCATE.
	(EMPTY_VAR_LIST): New define.
	(check_allocate_directive_restrictions): New function.
	(gfc_resolve_omp_allocate): Likewise.
	(gfc_resolve_omp_directive): Handle EXEC_OMP_ALLOCATE.
	* parse.cc (decode_omp_directive): Handle ST_OMP_ALLOCATE.
	(next_statement): Likewise.
	(gfc_ascii_statement): Likewise.
	* resolve.cc (gfc_resolve_code): Handle EXEC_OMP_ALLOCATE.
	* st.cc (gfc_free_statement): Likewise.
	* trans.cc (trans_code): Likewise