Do not call cp_parser_omp_dispatch directly in cp_parser_pragma

This is a followup to
ed49709a OpenMP: C++ front-end support for dispatch + adjust_args.

The call to cp_parser_omp_dispatch only belongs in cp_parser_omp_construct. In
cp_parser_pragma, handle PRAGMA_OMP_DISPATCH by calling cp_parser_omp_construct.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_pragma): Replace call to cp_parser_omp_dispatch
	with cp_parser_omp_construct and check context.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/dispatch-8.C: New test.