Fortran: Implement OpenMP 5.1 scope construct

Fortran version to commit e45483c7,
which implemented OpenMP's scope construct for C and C++.
Most testcases are based on the C testcases; it also contains some
testcases which existed previously but had no Fortran equivalent.

gcc/fortran/ChangeLog:

	* dump-parse-tree.c (show_omp_node, show_code_node): Handle
	EXEC_OMP_SCOPE.
	* gfortran.h (enum gfc_statement): Add ST_OMP_(END_)SCOPE.
	(enum gfc_exec_op): Add EXEC_OMP_SCOPE.
	* match.h (gfc_match_omp_scope): New.
	* openmp.c (OMP_SCOPE_CLAUSES): Define
	(gfc_match_omp_scope): New.
	(gfc_match_omp_cancellation_point, gfc_match_omp_end_nowait):
	Improve error diagnostic.
	(omp_code_to_statement): Handle ST_OMP_SCOPE.
	(gfc_resolve_omp_directive): Handle EXEC_OMP_SCOPE.
	* parse.c (decode_omp_directive, next_statement,
	gfc_ascii_statement, parse_omp_structured_block,
	parse_executable): Handle OpenMP's scope construct.
	* resolve.c (gfc_resolve_blocks): Likewise
	* st.c (gfc_free_statement): Likewise
	* trans-openmp.c (gfc_trans_omp_scope): New.
	(gfc_trans_omp_directive): Call it.
	* trans.c (trans_code): handle EXEC_OMP_SCOPE.

libgomp/ChangeLog:

	* testsuite/libgomp.fortran/scope-1.f90: New test.
	* testsuite/libgomp.fortran/task-reduction-16.f90: New test.

gcc/testsuite/ChangeLog:

	* gfortran.dg/gomp/scan-1.f90:
	* gfortran.dg/gomp/cancel-1.f90: New test.
	* gfortran.dg/gomp/cancel-4.f90: New test.
	* gfortran.dg/gomp/loop-4.f90: New test.
	* gfortran.dg/gomp/nesting-1.f90: New test.
	* gfortran.dg/gomp/nesting-2.f90: New test.
	* gfortran.dg/gomp/nesting-3.f90: New test.
	* gfortran.dg/gomp/nowait-1.f90: New test.
	* gfortran.dg/gomp/reduction-task-1.f90: New test.
	* gfortran.dg/gomp/reduction-task-2.f90: New test.
	* gfortran.dg/gomp/reduction-task-2a.f90: New test.
	* gfortran.dg/gomp/reduction-task-3.f90: New test.
	* gfortran.dg/gomp/scope-1.f90: New test.
	* gfortran.dg/gomp/scope-2.f90: New test.