PR libstdc++/88738
        Warn about unused comparisons of shared_ptr/unique_ptr
        * include/bits/c++config [_GLIBCXX_NODISCARD]: Define.
        * include/bits/shared_ptr.h: Use it for operator ==, !=,
        <, <=, >, >= for shared_ptr.
        * include/bits/unique_ptr.h: Likewise for unique_ptr.

From-SVN: r267964