libstdc++: add constexpr inplace_merge

libstdc++: add constexpr inplace_merge

This commit adds support for constexpr inplace_merge, added by P2562R1
for C++26. The implementation strategy is the same as for constexpr
stable_sort: use if consteval to detect if we're in constant evaluation,
and dispatch to a suitable path (same one as freestanding).

libstdc++-v3/ChangeLog:

	* include/bits/algorithmfwd.h (inplace_merge): Mark it as
	constexpr for C++26.
	* include/bits/ranges_algo.h (__inplace_merge_fn): Likewise.
	* include/bits/stl_algo.h (inplace_merge): Mark it as constexpr;
	during constant evaluation, dispatch to the non-allocating
	codepath.
	* testsuite/25_algorithms/headers/algorithm/synopsis.cc
	(inplace_merge): Add constexpr.
	* testsuite/25_algorithms/inplace_merge/constexpr.cc: New test.