openmp: Fix up firstprivate+lastprivate clause handling [PR99928]

The C/C++ clause splitting happens very early during construct parsing,
but only the FEs later on handle possible instantiations, non-static
member handling and array section lowering.
In the OpenMP 5.0/5.1 rules, whether firstprivate is added to combined
target depends on whether it isn't also mentioned in lastprivate or map
clauses, but unfortunately I think such checks are much better done only
when the FEs perform all the above mentioned changes.
So, this patch arranges for the firstprivate clause to be copied or moved
to combined target construct (as before), but sets flags on that clause,
which tell the FE *finish_omp_clauses and the gimplifier it has been added
only conditionally and let the FEs and gimplifier DTRT for these.

2021-05-21  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/99928
gcc/
	* tree.h (OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT_TARGET): Define.
	* gimplify.c (enum gimplify_omp_var_data): Fix up
	GOVD_MAP_HAS_ATTACHMENTS value, add GOVD_FIRSTPRIVATE_IMPLICIT.
	(omp_lastprivate_for_combined_outer_constructs): If combined target
	has GOVD_FIRSTPRIVATE_IMPLICIT set for the decl, change it to
	GOVD_MAP | GOVD_SEEN.
	(gimplify_scan_omp_clauses): Set GOVD_FIRSTPRIVATE_IMPLICIT for
	firstprivate clauses with OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT.
	(gimplify_adjust_omp_clauses): For firstprivate clauses with
	OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT either clear that bit and
	OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT_TARGET too, or remove it and
	let it be replaced by implicit map clause.
gcc/c-family/
	* c-omp.c (c_omp_split_clauses): Set OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT
	on firstprivate clause copy going to target construct, and for
	target simd set also OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT_TARGET bit.
gcc/c/
	* c-typeck.c (c_finish_omp_clauses): Move firstprivate clauses with
	OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT to the end of the chain.  Don't error
	if a decl is mentioned both in map clause and in such firstprivate
	clause unless OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT_TARGET is also set.
gcc/cp/
	* semantics.c (finish_omp_clauses): Move firstprivate clauses with
	OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT to the end of the chain.  Don't error
	if a decl is mentioned both in map clause and in such firstprivate
	clause unless OMP_CLAUSE_FIRSTPRIVATE_IMPLICIT_TARGET is also set.
gcc/testsuite/
	* c-c++-common/gomp/pr99928-3.c: Remove all xfails.
	* c-c++-common/gomp/pr99928-15.c: New test.