Merge 79729 from mainline.

  StringRef str() const { return StringRef(this->begin(), this->size()); }

  // Extra operators.

  const SmallString &operator=(const char *RHS) {

  const SmallString &operator=(StringRef RHS) {

    this->clear();

    return *this += RHS;

  }

  SmallString &operator+=(const char *RHS) {

    this->append(RHS, RHS+strlen(RHS));

  SmallString &operator+=(StringRef RHS) {

    this->append(RHS.begin(), RHS.end());

    return *this;

  }

  SmallString &operator+=(char C) {