OpenMP: Fortran support for imperfectly-nested loops

OpenMP 5.0 removed the restriction that multiple collapsed loops must
be perfectly nested, allowing "intervening code" (including nested
BLOCKs) before or after each nested loop.  In GCC this code is moved
into the inner loop body by the respective front ends.

In the Fortran front end, most of the semantic processing happens during
the translation phase, so the parse phase just collects the intervening
statements, checks them for errors, and splices them around the loop body.

gcc/fortran/ChangeLog
	* gfortran.h (struct gfc_namespace): Add omp_structured_block bit.
	* openmp.cc: Include omp-api.h.
	(resolve_omp_clauses): Consolidate inscan reduction clause conflict
	checking here.
	(find_nested_loop_in_chain): New.
	(find_nested_loop_in_block): New.
	(gfc_resolve_omp_do_blocks): Set omp_current_do_collapse properly.
	Handle imperfectly-nested loops when looking for nested omp scan.
	Refactor to move inscan reduction clause conflict checking to
	resolve_omp_clauses.
	(gfc_resolve_do_iterator): Handle imperfectly-nested loops.
	(struct icode_error_state): New.
	(icode_code_error_callback): New.
	(icode_expr_error_callback): New.
	(diagnose_intervening_code_errors_1): New.
	(diagnose_intervening_code_errors): New.
	(make_structured_block): New.
	(restructure_intervening_code): New.
	(is_outer_iteration_variable): Do not assume loops are perfectly
	nested.
	(check_nested_loop_in_chain): New.
	(check_nested_loop_in_block_state): New.
	(check_nested_loop_in_block_symbol): New.
	(check_nested_loop_in_block): New.
	(expr_uses_intervening_var): New.
	(is_intervening_var): New.
	(expr_is_invariant): Do not assume loops are perfectly nested.
	(resolve_omp_do): Handle imperfectly-nested loops.
	* trans-stmt.cc (gfc_trans_block_construct): Generate
	OMP_STRUCTURED_BLOCK if magic bit is set on block namespace.

gcc/testsuite/ChangeLog
	* gfortran.dg/gomp/collapse1.f90: Adjust expected errors.
	* gfortran.dg/gomp/collapse2.f90: Likewise.
	* gfortran.dg/gomp/imperfect-gotos.f90: New.
	* gfortran.dg/gomp/imperfect-invalid-scope.f90: New.
	* gfortran.dg/gomp/imperfect1.f90: New.
	* gfortran.dg/gomp/imperfect2.f90: New.
	* gfortran.dg/gomp/imperfect3.f90: New.
	* gfortran.dg/gomp/imperfect4.f90: New.
	* gfortran.dg/gomp/imperfect5.f90: New.

libgomp/ChangeLog
	* testsuite/libgomp.fortran/imperfect-destructor.f90: New.
	* testsuite/libgomp.fortran/imperfect1.f90: New.
	* testsuite/libgomp.fortran/imperfect2.f90: New.
	* testsuite/libgomp.fortran/imperfect3.f90: New.
	* testsuite/libgomp.fortran/imperfect4.f90: New.
	* testsuite/libgomp.fortran/target-imperfect1.f90: New.
	* testsuite/libgomp.fortran/target-imperfect2.f90: New.
	* testsuite/libgomp.fortran/target-imperfect3.f90: New.
	* testsuite/libgomp.fortran/target-imperfect4.f90: New.