/// opening a string literal, advances CurPtr if a delimiter is found and
/// returns a non-zero delimiter length. CurPtr[-1] must be '#' when called.
static unsigned advanceIfCustomDelimiter(const char *&CurPtr,
                                         DiagnosticEngine *Diags) {
  assert(CurPtr[-1] == '#');

  const char *TmpPtr = CurPtr;