Make std::enable_shared_from_this cope with ambiguity

Make std::enable_shared_from_this cope with ambiguity

	* include/backward/auto_ptr.h (__shared_ptr(auto_ptr&&)): Call
	_M_enable_shared_from_this_with instead of
	__enable_shared_from_this_helper.
	* include/bits/shared_ptr.h (__enable_shared_from_this_helper):
	Remove overload for std::enable_shared_from_this..
	(__enable_shared_from_this_base): Define friend function to select a
	std::enable_shared_from_this base class.
	* include/bits/shared_ptr_base.h (__enable_shared_from_this_helper):
	Remove all overloads.
	(__shared_ptr): Change all relevant constructors to call
	_M_enable_shared_from_this_with instead of
	__enable_shared_from_this_helper.
	(__shared_ptr::__efst_base_t, __shared_ptr::__has_efst_base): Helpers
	to detect accessible and unambiguous enable_shared_from_this bases.
	(__shared_ptr::_M_enable_shared_from_this_with): New function to
	replace __enable_shared_from_this_helper overloads.
	(__enable_shared_from_this_helper): Remove overload for
	std::__enable_shared_from_this.
	(__enable_shared_from_this_base): Define friend function to select a
	std::__enable_shared_from_this base class.
	* include/experimental/bits/shared_ptr.h (experimental::shared_ptr):
	Change relevant constructors to call _M_enable_shared_from_this_with.
	(experimental::shared_ptr::__efst_base_t)
	(experimental::shared_ptr::__has_efst_base): Helpers to detect
	accessible and unambiguous enable_shared_from_this bases.
	(experimental::shared_ptr::_M_enable_shared_from_this_with): Define.
	(experimental::__enable_shared_from_this_helper): Remove overload for
	std::experimental::enable_shared_from_this.
	(experimental::__expt_enable_shared_from_this_base): Define friend
	function to select a std::experimental::enable_shared_from_this base.
	* testsuite/experimental/memory/shared_ptr/cons/
	enable_shared_from_this.cc: New test.
	* testsuite/experimental/memory/shared_ptr/cons/unique_ptr_ctor.cc:
	Adjust expected behaviour for shared_ptr<A[]>.

From-SVN: r241353