libstdc++: Define __cpp_lib_constexpr_algorithms in <utility> (LWG 3792)

We actually defined this macro in <utility> at one point, but I removed
it in r10-7901-g2025db692e9ed1.

libstdc++-v3/ChangeLog:

	* include/std/utility (__cpp_lib_constexpr_algorithms): Define,
	as per LWG 3792.
	* testsuite/20_util/exchange/constexpr.cc: Check for it.