message: Align the cached value in FlatMap's iterator.
  This may be misaligned for types requiring >=8-byte alignment (like pointer
  types).
diff: @@ -146,8 +146,8 @@ class FlatMap {
  friend class FlatMap;
  Bucket* b_;
  Bucket* end_;
  + char space_ alignas(value_type)[sizeof(value_type)];
  uint32 i_;
  - char space_[sizeof(value_type)];
  pointer val() { return reinterpret_cast<pointer>(space_); }
  void FillValue() { new (space_) value_type(b_->key(i_), b_->val(i_)); }
  