openmp: Add support for strict modifier on grainsize/num_tasks clauses

openmp: Add support for strict modifier on grainsize/num_tasks clauses

With strict: modifier on these clauses, the standard is explicit about
how many iterations (and which) each generated task of taskloop directive
should contain.  For num_tasks it actually matches what we were already
implementing, but for grainsize it does not (and even violates the old
rule - without strict it requires that the number of iterations (unspecified
which exactly) handled by each generated task is >= grainsize argument and
< 2 * grainsize argument, with strict: it requires that each generated
task handles exactly == grainsize argument iterations, except for the
generated task handling the last iteration which can handles <= grainsize
iterations).

The following patch implements it for C and C++.

2021-08-23  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* tree.h (OMP_CLAUSE_GRAINSIZE_STRICT): Define.
	(OMP_CLAUSE_NUM_TASKS_STRICT): Define.
	* tree-pretty-print.c (dump_omp_clause) <case OMP_CLAUSE_GRAINSIZE,
	case OMP_CLAUSE_NUM_TASKS>: Print strict: modifier.
	* omp-expand.c (expand_task_call): Use GOMP_TASK_FLAG_STRICT in iflags
	if either grainsize or num_tasks clause has the strict modifier.
gcc/c/
	* c-parser.c (c_parser_omp_clause_num_tasks,
	c_parser_omp_clause_grainsize): Parse the optional strict: modifier.
gcc/cp/
	* parser.c (cp_parser_omp_clause_num_tasks,
	cp_parser_omp_clause_grainsize): Parse the optional strict: modifier.
include/
	* gomp-constants.h (GOMP_TASK_FLAG_STRICT): Define.
libgomp/
	* taskloop.c (GOMP_taskloop): Handle GOMP_TASK_FLAG_STRICT.
	* testsuite/libgomp.c-c++-common/taskloop-4.c (main): Fix up comment.
	* testsuite/libgomp.c-c++-common/taskloop-5.c: New test.