re PR libstdc++/77537 (pair constructors do not properly SFINAE)

re PR libstdc++/77537 (pair constructors do not properly SFINAE)

	PR libstdc++/77537
	Implement LWG 2729 for pair.
	* include/bits/stl_pair.h (_PCC): New.
	(_ConstructiblePair, _ImplicitlyConvertiblePair):
	Turn into static member functions of _PCC.
	(_MoveConstructiblePair, _ImplicitlyMoveConvertiblePair): Likewise.
	(_PCCP): New.
	(pair(const _T1&, const _T2&)): Adjust.
	(_PCCFP): New.
	(pair(const pair<_U1, _U2>&)): Adjust.
	(pair(_U1&&, const _T2&)): Likewise.
	(pair(const _T1&, _U2&&)): Likewise.
	(pair(_U1&&, _U2&&)): Likewise.
	(pair(pair<_U1, _U2>&&)): Likewise.
	(operator=(const pair&)): Make conditionally deleted.
	(operator=(pair&&)): Make conditionally suppressed.
	(operator=(const pair<_U1, _U2>&)): Constrain.
	(operator=(pair<_U1, _U2>&&): Likewise.
	* include/std/type_traits (__nonesuch): New.
	* testsuite/20_util/pair/traits.cc: New.

From-SVN: r240322