Implement sane variant converting constructor (P0608R3)

Implement sane variant converting constructor (P0608R3)

	* include/std/variant (__overload_set): Remove.
	(_Arr): New helper.
	(_Build_FUN): New class template to define a single FUN overload,
	with specializations to prevent unwanted conversions, as per P0608R3.
	(_Build_FUNs): New class template to build an overload set of FUN.
	(_FUN_type): New alias template to perform overload resolution.
	(__accepted_type): Use integer_constant base for failure case. Use
	_FUN_type for successful case.
	(variant::__accepted_index): Use _Tp instead of _Tp&&.
	(variant::variant(_Tp&&)): Likewise.
	(variant::operator=(_Tp&&)): Likewise.

From-SVN: r271296