[TableGen] Use std::string::find (NFC) (#139681)
This patch partially reverts #139661 for a better solution.
Specifically, we can take advantage of the fact that std::string::find
accepts anything that can be converted to std::string_view, including
StringRef, starting with C++17. This way, we do not need to cast Val
to StringRef or LHSs->getValue() to std::string.
