c++: Fix up detach clause vs. data-sharing clause checking [PR100319]

c++: Fix up detach clause vs. data-sharing clause checking [PR100319]

The standard says that "The event-handle will be considered as if it
was specified on a firstprivate clause." which means that it can't
be explicitly specified in some other data-sharing clause.
The checking is implemented correctly for C, but for C++ when detach_seen
is true (i.e. the construct had detach clause) we were comparing
OMP_CLAUSE_DECL (c) with t, which was previously initialized to
OMP_CLAUSE_DECL (c), which means it complained about any explicit
data-sharing clause on the same construct with a detach clause.

Fixed by remembering the detach clause in detach_seen (instead of a boolean
flag) and comparing against its OMP_CLAUSE_DECL.

2021-04-29  Jakub Jelinek  <jakub@redhat.com>

	PR c++/100319
	* semantics.c (finish_omp_clauses): Fix up check that variable
	mentioned in detach clause doesn't appear in data-sharing clauses.

	* c-c++-common/gomp/task-detach-3.c: New test.