message: Use StrAppend instead of ostringstream
diff: #include <string>
  #include <vector>
  +#include "absl/strings/str_cat.h"
  #include "absl/strings/string_view.h"
  #include "common/strings/line_column_map.h"
  #include "common/text/concrete_syntax_leaf.h"
  namespace verible {
  std::string AutoFix::Apply(const absl::string_view base) const {
  - std::ostringstream result;
  -
  + std::string result;
  auto prev_start = base.cbegin();
  for (const auto& edit : edits) {
  CHECK_LE(base.cbegin(), edit.fragment.cbegin());
  CHECK_GE(base.cend(), edit.fragment.cend());
  - absl::string_view text_before = absl::string_view(
  + const absl::string_view text_before(
  prev_start, std::distance(prev_start, edit.fragment.cbegin()));
  + absl::StrAppend(&result, text_before, edit.replacement);
  - result << text_before << edit.replacement;
  prev_start = edit.fragment.cend();
  }
  - absl::string_view text_after =
  - absl::string_view(prev_start, std::distance(prev_start, base.cend()));
  - result << text_after;
  - return result.str();
  + const absl::string_view text_after(prev_start,
  + std::distance(prev_start, base.cend()));
  + return absl::StrCat(result, text_after);
  }
  bool AutoFix::AddEdits(const std::set<ReplacementEdit>& new_edits) {
  