libstdc++: Fix resolution of LWG 4016 for std::ranges::to [PR112876]

What I implemented in r14-6199-g45630fbcf7875b does not match what I
proposed for LWG 4016, and it imposes additional, unwanted requirements
on the emplace and insert member functions of the container being
populated.

libstdc++-v3/ChangeLog:

	PR libstdc++/112876
	* include/std/ranges (ranges::to): Do not try to use an iterator
	returned by the container's emplace or insert member functions.
	* testsuite/std/ranges/conv/1.cc (Cont4::emplace, Cont4::insert):
	Use the iterator parameter. Do not return an iterator.