PR libstdc++/51365
	* include/bits/shared_ptr_base (_Sp_ebo_helper): Helper class to
	implement EBO safely.
	(_Sp_counted_base::_M_get_deleter): Add noexcept.
	(_Sp_counter_ptr): Use noexcept instead of comments.
	(_Sp_counted_deleter): Likewise. Use _Sp_ebo_helper.
	(_Sp_counted_ptr_inplace): Likewise.
	* testsuite/20_util/shared_ptr/cons/51365.cc: New.
	* testsuite/20_util/shared_ptr/cons/52924.cc: Add rebind member to
	custom allocator and test construction with custom allocator.
	* testsuite/20_util/shared_ptr/cons/43820_neg.cc: Adjust dg-error
	line number.

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@198367 138bc75d-0d04-0410-961f-82ee72b054a4