'omp scan' struct block seq update for OpenMP 5.x

'omp scan' struct block seq update for OpenMP 5.x

While OpenMP 5.0 required a single structured block before and after the
'omp scan' directive, OpenMP 5.1 changed this to a 'structured block sequence,
denoting 2 or more executable statements in OpenMP 5.1 (whoops!) and zero or
more in OpenMP 5.2. This commit updates C/C++ to accept zero statements (but
till requires the '{' ... '}' for the final-loop-body) and updates Fortran
to accept zero or more than one statements.

If there is no preceeding or succeeding executable statement, a warning is
shown.

gcc/c/ChangeLog:

	* c-parser.cc (c_parser_omp_scan_loop_body): Handle
	zero exec statements before/after 'omp scan'.

gcc/cp/ChangeLog:

	* parser.cc (cp_parser_omp_scan_loop_body): Handle
	zero exec statements before/after 'omp scan'.

gcc/fortran/ChangeLog:

	* openmp.cc (gfc_resolve_omp_do_blocks): Handle zero
	or more than one exec statements before/after 'omp scan'.
	* trans-openmp.cc (gfc_trans_omp_do): Likewise.

libgomp/ChangeLog:

	* testsuite/libgomp.c-c++-common/scan-1.c: New test.
	* testsuite/libgomp.c/scan-23.c: New test.
	* testsuite/libgomp.fortran/scan-2.f90: New test.

gcc/testsuite/ChangeLog:

	* g++.dg/gomp/attrs-7.C: Update dg-error/dg-warning.
	* gfortran.dg/gomp/loop-2.f90: Likewise.
	* gfortran.dg/gomp/reduction5.f90: Likewise.
	* gfortran.dg/gomp/reduction6.f90: Likewise.
	* gfortran.dg/gomp/scan-1.f90: Likewise.
	* gfortran.dg/gomp/taskloop-2.f90: Likewise.
	* c-c++-common/gomp/scan-6.c: New test.
	* gfortran.dg/gomp/scan-8.f90: New test.