libstdc++: Fix constraints for std::variant default constructor

libstdc++: Fix constraints for std::variant default constructor

The standard says the default ctor should be constrained, not deleted.
Our use of a defaulted default ctor and _Enable_default_constructor base
class results in it being deleted.

libstdc++-v3/ChangeLog:

	* include/std/variant (variant): Remove derivation from
	_Enable_default_constructor base class.
	(variant::variant()): Constrain.
	* testsuite/20_util/variant/default_ctor.cc: New test.