libstdc++: Allow unordered_set assignment to assign to existing nodes

libstdc++: Allow unordered_set assignment to assign to existing nodes

Currently the _ReuseOrAllocNode::operator(Args&&...) function always
destroys the value stored in recycled nodes and constructs a new value.

The _ReuseOrAllocNode type is only ever used for implementing
assignment, either from another unordered container of the same type, or
from std::initializer_list<value_type>. Consequently, the parameter pack
Args only ever consists of a single parameter or type const value_type&
or value_type.  We can replace the variadic parameter pack with a single
forwarding reference parameter, and when the value_type is assignable
from that type we can use assignment instead of destroying the existing
value and then constructing a new one.

Using assignment is typically only possible for sets, because for maps
the value_type is std::pair<const key_type, mapped_type> and in most
cases std::is_assignable_v<const key_type&, const key_type&> is false.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable_policy.h (_ReuseOrAllocNode::operator()):
	Replace parameter pack with a single parameter. Assign to
	existing value when possible.
	* testsuite/23_containers/unordered_multiset/allocator/move_assign.cc:
	Adjust expected count of operations.
	* testsuite/23_containers/unordered_set/allocator/move_assign.cc:
	Likewise.

Reviewed-by: François Dumont <fdumont@gcc.gnu.org>