This patch adds memoization to these four views so that their begin() has the
required amortized constant time complexity.

The cache is enabled only for forward_ranges and above because we need the
underlying iterator to be copyable and multi-pass in order for the cache to be
usable.  In the general case we represent the cached result of begin() as a bare
iterator.  This takes advantage of the fact that value-initialized forward
iterators can be compared to as per N3644, so we can use a value-initialized
iterator to denote the "empty" state of the cache.

As a special case, when the underlying range models random_access_range and when
it's profitable size-wise, then we cache the offset of the iterator from the
beginning of the range instead of caching the iterator itself.

Additionally, in drop_view and reverse_view we disable the cache when the
underlying range models random_access_range, because in these cases recomputing
begin() takes O(1) time anyway.

libstdc++-v3/ChangeLog:

	* include/std/ranges (__detail::_CachedPosition): New struct.
	(views::filter_view::_S_needs_cached_begin): New member variable.
	(views::filter_view::_M_cached_begin): New member variable.
	(views::filter_view::begin): Use _M_cached_begin to cache its
	result.
	(views::drop_view::_S_needs_cached_begin): New static member variable.
	(views::drop_view::_M_cached_begin): New member variable.
	(views::drop_view::begin): Use _M_cached_begin to cache its result
	when _S_needs_cached_begin.
	(views::drop_while_view::_M_cached_begin): New member variable.
	(views::drop_while_view::begin): Use _M_cached_begin to cache its
	result.
	(views::reverse_view::_S_needs_cached_begin): New static member
	variable.
	(views::reverse_view::_M_cached_begin): New member variable.
	(views::reverse_view::begin): Use _M_cached_begin to cache its result
	when _S_needs_cached_begin.
	* testsuite/std/ranges/adaptors/drop.cc: Augment test to check that
	drop_view::begin caches its result.
	* testsuite/std/ranges/adaptors/drop_while.cc: Augment test to check
	that drop_while_view::begin caches its result.
	* testsuite/std/ranges/adaptors/filter.cc: Augment test to check that
	filter_view::begin caches its result.
	* testsuite/std/ranges/adaptors/reverse.cc: Augment test to check that
	reverse_view::begin caches its result.