libstdc++: Do not over-size hashtable buckets on range insertion

libstdc++: Do not over-size hashtable buckets on range insertion

We used to consider range size on insertion but on unique keys container
not all range values might be inserted resulting in over-sizing. In this
case we just consider user reservation and if none then the container will
adapt to actually inserted elements.

libstdc++-v3/ChangeLog:

	* include/bits/hashtable.h
	(_Hashtable<>(_InputIterator, _InputIterator, size_t, const _H1&,
	const _H2&, const _Hash&, const _Equal&, const _ExtractKey&,
	const allocator_type&, true_type)): New.
	(_Hashtable<>(_InputIterator, _InputIterator, size_t, const _H1&,
	const _H2&, const _Hash&, const _Equal&, const _ExtractKey&,
	const allocator_type&, false_type)): New.
	(_Hashtable<>(_InputIterator, _InputIterator, size_t, const _H1&,
	const _H2&, const _Hash&, const _Equal&, const _ExtractKey&,
	const allocator_type&)): Delegate to latters.
	(operator=(initializer_list<value_type>)): Rehash if too small.
	(_M_insert(_Arg&&, const _NodeGenerator&, true_type)): Remove
	size_t len parameter.
	* include/bits/hashtable_policy.h (_Insert_base<>::_M_insert_range):
	Do not try to get input range distance.
	* testsuite/23_containers/unordered_set/cons/bucket_hint.cc: New test.
	* testsuite/23_containers/unordered_set/modifiers/insert.cc: New test.