libstdc++: Implement P2017R1 "Conditionally borrowed ranges"

libstdc++: Implement P2017R1 "Conditionally borrowed ranges"

This makes some range adaptors model the borrowed_range concept if they
are adapting a borrowed range. This hasn't been added to the C++23
working paper yet, but it has been approved by LWG, and the
recommendation is to treat it as a defect report for C++20 as well.

libstdc++-v3/ChangeLog:

	* include/std/ranges (enable_borrowed_view<take_view<T>>)
	(enable_borrowed_view<drop_view<T>>)
	(enable_borrowed_view<drop_while_view<T>>)
	(enable_borrowed_view<reverse_view<T>>)
	(enable_borrowed_view<common_view<T>>)
	(enable_borrowed_view<elements_view<T>>): Add partial
	specializations as per P2017R1.
	* testsuite/std/ranges/adaptors/conditionally_borrowed.cc:
	New test.