libstdc++: Refactor _Hashtable::operator=(initializer_list<value_type>)

This replaces a call to _M_insert_range with open coding the loop. This
will allow removing the node generator parameter from _M_insert_range in
a later commit.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h (operator=(initializer_list)):
	Refactor to not use _M_insert_range.

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