libstdc++: Make ranges::construct_at constexpr-friendly [PR95788]

libstdc++: Make ranges::construct_at constexpr-friendly [PR95788]

This rewrites ranges::construct_at in terms of std::construct_at so
that we can piggyback on the compiler's existing support for
intercepting placement new within std::construct_at during constexpr
evaluation, instead of having to additionally teach the compiler about
ranges::construct_at.

While we're making changes to ranges::construct_at, this patch also
declares it conditionally noexcept and qualifies the calls to declval in
its requires-clause.

libstdc++-v3/ChangeLog:

	PR libstdc++/95788
	* include/bits/ranges_uninitialized.h:
	(__construct_at_fn::operator()): Rewrite in terms of
	std::construct_at.  Declare it conditionally noexcept.  Qualify
	calls to declval in its requires-clause.
	* testsuite/20_util/specialized_algorithms/construct_at/95788.cc:
	New test.