libstdc++: Fix complexity of drop_view::begin() const [PR112641]

libstdc++: Fix complexity of drop_view::begin() const [PR112641]

Views are required to have a amortized O(1) begin(), but our drop_view's
const begin overload is O(n) for non-common ranges with a non-sized
sentinel.  This patch reimplements it so that it's O(1) always.  See
also LWG 4009.

	PR libstdc++/112641

libstdc++-v3/ChangeLog:

	* include/std/ranges (drop_view::begin): Reimplement const
	overload so that it's O(1) always.
	* testsuite/std/ranges/adaptors/drop.cc (test10): New test.

Reviewed-by: Jonathan Wakely <jwakely@redhat.com>
(cherry picked from commit 7f622ee83fbbcf4a4ca70e020db8a0ce4b556b61)