message: Prevent unnecessary conversion from StringRef to C-string [NFC]
  There is an alternative method to GetConstCStringWithLength that
  takes a StringRef. GetConstCStringWithLength also calls this
  method in the end, so directly calling the StringRef saves
  us from a unnecessary conversion to a C-string.
diff: @@ -200,7 +200,7 @@ ConstString::ConstString(const char *cstr, size_t cstr_len)
  : m_string(StringPool().GetConstCStringWithLength(cstr, cstr_len)) {}
  ConstString::ConstString(const llvm::StringRef &s)
  - : m_string(StringPool().GetConstCStringWithLength(s.data(), s.size())) {}
  + : m_string(StringPool().GetConstCStringWithStringRef(s)) {}
  bool ConstString::operator<(ConstString rhs) const {
  if (m_string == rhs.m_string)
  