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.
From-SVN: r198367
