message: Fix build with C++17
  Summary:
  In C++17, `emplace_back` returns a reference to the new element instead
  of void. This fixes a call where this results in returning a value from
  a void-returning function.
  Closes
diff: @@ -389,7 +389,7 @@ class DexAnnotationSet : public Gatherable {
  }
  std::vector<DexAnnotation*>& get_annotations() { return m_annotations; }
  void add_annotation(DexAnnotation* anno) {
  - return m_annotations.emplace_back(anno);
  + m_annotations.emplace_back(anno);
  }
  void vencode(DexOutputIdx* dodx,
  std::vector<uint32_t>& asetout,
  