Simplify node ownership in _Hashtable members

Simplify node ownership in _Hashtable members

Introduce an RAII type to manage nodes in unordered containers while
they are being inserted. If the caller always owns a node until it is
inserted, then the insertion functions don't need to deallocate on
failure. This allows a FIXME in the node re-insertion API to be removed.

Also change extract(const key_type&) to not call extract(const_iterator)
anymore.  This avoids looping through the bucket nodes again to find the
node before the one being extracted.

2019-06-17  François Dumont  <fdumont@gcc.gnu.org>
	    Jonathan Wakely  <jwakely@redhat.com>

	* include/bits/hashtable.h (struct _Hashtable::_Scoped_node): New type.
	(_Hashtable::_M_insert_unique_node): Add key_type parameter. Don't
	deallocate node if insertion fails.
	(_Hashtable::_M_insert_multi_node): Likewise.
	(_Hashtable::_M_reinsert_node): Pass additional key argument.
	(_Hashtable::_M_reinsert_node_multi): Likewise. Remove FIXME.
	(_Hashtable::_M_extract_node(size_t, __node_base*)): New function.
	(_Hashtable::extract(const_iterator)): Use _M_extract_node.
	(_Hashtable::extract(const _Key&)): Likewise.
	(_Hashtable::_M_merge_unique): Pass additional key argument.
	(_Hashtable::_M_emplace<Args>(true_type, Args&&...)): Likewise. Use
	_Scoped_node.
	(_Hashtable::_M_insert): Likewise.
	* include/bits/hashtable_policy.h (_Map_base::operator[]): Likewise.
	(_Hashtable_alloc): Add comments to functions with misleading names.

Co-Authored-By: Jonathan Wakely <jwakely@redhat.com>
From-SVN: r272381