openmp: Handle unconstrained and reproducible modifiers on order(concurrent)

This patch adds handling for unconstrained and reproducible modifiers on
order(concurrent) clause.  For all static schedules (including auto and
no schedule or dist_schedule clauses) I believe what we implement is
reproducible, so the patch doesn't do much beyond recognizing those.
Note, there is an OpenMP/spec issue that needs resolution on what
should happen with the dynamic schedules (whether it should be an error
to mix such clauses, or silently make it non-reproducible, and in which
exact cases), so it might need some follow-up.

Besides that, this patch allows order(concurrent) clause on the distribute
construct which is something also added in OpenMP 5.1, and finally
check the newly added restriction that at most one order clause
can appear on a construct.

The allowing of order clause on distribute has a side-effect that
order(concurrent) copyin(thrpriv) is no longer allowed on combined/composite
constructs with distribute parallel for{, simd} in it, previously the
order applied only to for/simd and so a threadprivate var could be seen
in the construct, but now it also applies to distribute and so on the parallel
we shouldn't refer to a threadprivate var.

2021-09-18  Jakub Jelinek  <jakub@redhat.com>

gcc/
	* tree.h (OMP_CLAUSE_ORDER_UNCONSTRAINED): Define.
	* tree-pretty-print.c (dump_omp_clause): Print unconstrained:
	for OMP_CLAUSE_ORDER_UNCONSTRAINED.
gcc/c-family/
	* c-omp.c (c_omp_split_clauses): Split order clause also to
	distribute construct.  Copy over OMP_CLAUSE_ORDER_UNCONSTRAINED.
gcc/c/
	* c-parser.c (c_parser_omp_clause_order): Parse unconstrained
	and reproducible modifiers.
	(OMP_DISTRIBUTE_CLAUSE_MASK): Add order clause.
gcc/cp/
	* parser.c (cp_parser_omp_clause_order): Parse unconstrained
	and reproducible modifiers.
	(OMP_DISTRIBUTE_CLAUSE_MASK): Add order clause.
gcc/testsuite/
	* c-c++-common/gomp/order-1.c (f2): Add tests for distribute
	with order clause.
	(f3): Remove.
	* c-c++-common/gomp/order-2.c: Don't expect error for distribute
	with order clause.
	* c-c++-common/gomp/order-5.c: New test.
	* c-c++-common/gomp/order-6.c: New test.
	* c-c++-common/gomp/clause-dups-1.c (f1): Add tests for
	duplicated order clause.
	(f9): New function.
	* c-c++-common/gomp/clauses-1.c (baz, bar): Don't mix copyin and
	order(concurrent) clauses on the same composite construct combined
	with distribute, instead split it into two tests, one without
	copyin and one without order(concurrent).  Add order(concurrent)
	clauses to {,{,target} teams} distribute.
	* g++.dg/gomp/attrs-1.C (baz, bar): Likewise.
	* g++.dg/gomp/attrs-2.C (baz, bar): Likewise.