Code refactoring of symbol_rewriter with modern C++ and new functions of unique_ptr<Candidate>.
Code refactoring of symbol_rewriter with modern C++ and new functions of unique_ptr<Candidate>.

PiperOrigin-RevId: 439483416

google/mozc,fcitx/mozc,google/mozc,google/mozc,google/mozc,fcitx/mozc,fcitx/mozc,google/mozc,fcitx/mozc,fcitx/mozc