Fortran: Add 'omp scan' support of OpenMP 5.0

Fortran: Add 'omp scan' support of OpenMP 5.0

gcc/fortran/ChangeLog:

	* dump-parse-tree.c (show_omp_clauses, show_omp_node,
	show_code_node): Handle OMP SCAN.
	* gfortran.h (enum gfc_statement): Add ST_OMP_SCAN.
	(enum): Add OMP_LIST_SCAN_IN and OMP_LIST_SCAN_EX.
	(enum gfc_exec_op): Add EXEC_OMP_SCAN.
	* match.h (gfc_match_omp_scan): New prototype.
	* openmp.c (gfc_match_omp_scan): New.
	(gfc_match_omp_taskgroup): Cleanup.
	(resolve_omp_clauses, gfc_resolve_omp_do_blocks,
	omp_code_to_statement, gfc_resolve_omp_directive): Handle 'omp scan'.
	* parse.c (decode_omp_directive, next_statement,
	gfc_ascii_statement): Likewise.
	* resolve.c (gfc_resolve_code): Handle EXEC_OMP_SCAN.
	* st.c (gfc_free_statement): Likewise.
	* trans-openmp.c (gfc_trans_omp_clauses, gfc_trans_omp_do,
	gfc_split_omp_clauses): Handle 'omp scan'.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/scan-1.f90: New test.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/reduction4.f90: Update; move FE some tests to ...
	* gfortran.dg/gomp/reduction6.f90: ... this new test and ...
	* gfortran.dg/gomp/reduction7.f90: ... this new test.
	* gfortran.dg/gomp/reduction5.f90: Add dg-error.
	* gfortran.dg/gomp/scan-1.f90: New test.
	* gfortran.dg/gomp/scan-2.f90: New test.
	* gfortran.dg/gomp/scan-3.f90: New test.
	* gfortran.dg/gomp/scan-4.f90: New test.
	* gfortran.dg/gomp/scan-5.f90: New test.
	* gfortran.dg/gomp/scan-6.f90: New test.
	* gfortran.dg/gomp/scan-7.f90: New test.