re PR libstdc++/58659 (Construction of shared_ptr from unique_ptr mismatches new/delete and std::allocator for __shared_ptr_count)

	PR libstdc++/58659
	* include/bits/shared_ptr_base.h (__shared_count::__shared_count(P,D)):
	Delegate to constructor taking allocator.
	(__shared_count::_S_create_from_up): Inline into ...
	(__shared_count::__shared_count(unique_ptr<Y,D>&&): Here. Use
	std::conditional instead of constrained overloads. Allocate memory
	using the allocator type that will be used for deallocation.
	* testsuite/20_util/shared_ptr/cons/58659.cc: New.
	* testsuite/20_util/shared_ptr/cons/43820_neg.cc: Adjust.

From-SVN: r203274