OpenMP/C++: Store location in cp_parser_omp_var_list for kind=0

OpenMP/C++: Store location in cp_parser_omp_var_list for kind=0

cp_parser_omp_var_list and cp_parser_omp_var_list_no_open have a special
modus: kind = 0 alias kind = OMP_CLAUSE_ERROR, which returns a simple tree
list; however, for a decl, no location is associated with that variable,
yielding to confusing error locations. With this patch, also for kind=0,
a reasonable error location is stored, albeit with creating a tree node
(build_empty_stmt), which is otherwise not used.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_omp_var_list_no_open,
	cp_parser_omp_var_list): For kind=0 (= OMP_CLAUSE_ERROR),
	store also the expression location in the tree list.
	(cp_parser_oacc_data_clause_deviceptr,
	cp_finish_omp_declare_variant): Use that location instead or
	input_location/the before-parsing location.
	* semantics.cc (finish_omp_threadprivate): Likewise.