Fix std::experimental::shared_ptr SFINAE constraints

Fix std::experimental::shared_ptr SFINAE constraints

	* include/experimental/bits/shared_ptr.h
	(__shared_ptr<__libfund_v1<_Tp, false>, _Lp>::_Compatible): Just use
	is_convertible for non-array specialization.
	(__shared_ptr<__libfund_v1<_Tp, false>, _Lp>::_UniqCompatible): New
	constraint for conversions from unique_ptr.
	(__shared_ptr<__libfund_v1<_Tp, false>, _Lp>::__shared_ptr): Constrain.
	(__shared_ptr<__libfund_v1<_Tp, false>, _Lp>::reset): Likewise.
	(__sp_compatible_v): New variable template for trait.
	(__sp_is_constructible): New trait to check shared_ptr constraints.
	(__sp_is_constructible_v): New variable template for trait.
	(__shared_ptr<__libfund_v1<_Tp, true>, _Lp>::_SafeConv): New
	constraint for construction/reset, using __sp_is_constructible_v.
	(__shared_ptr<__libfund_v1<_Tp, true>, _Lp>::_UniqCompatible): New
	constraint for conversions from unique_ptr.
	(__shared_ptr<__libfund_v1<_Tp, true>, _Lp>::__shared_ptr): Constrain.
	(__shared_ptr<__libfund_v1<_Tp, true>, _Lp>::reset): Likewise.
	(shared_ptr::_SafeConv): Constraint for checking constructors.
	(shared_ptr(Tp1*), shared_ptr(_Tp1, _Deleter))
	(shared_ptr(_Tp1, _Deleter, _Alloc)): Constrain with _SafeConv.
	(shared_ptr(const weak_ptr<_Tp1>&)): Constrain with _Compatible.
	(shared_ptr(auto_ptr<_Tp1>&&)): Fix, remove TODO.
	* testsuite/experimental/memory/shared_ptr/cons/pointer_ctor.cc:
	Remove tests using invalid conversions.
	* testsuite/experimental/memory/shared_ptr/cons/pointer_ctor_neg.cc:
	New test.
	* testsuite/experimental/memory/shared_ptr/cons/torture.cc: New test.
	* testsuite/experimental/memory/shared_ptr/modifiers/reset.cc: Remove
	tests using invalid conversions.
	* testsuite/experimental/memory/shared_ptr/modifiers/reset_neg.cc: New
	test.
	* testsuite/experimental/memory/shared_ptr/observers/use_count.cc:
	Remove tests using invalid conversions.

From-SVN: r241331