[llvm] [NFC][TableGen] Use StringRef::str() instead of casting (PR #139332)

[Kazu Hirata via llvm-commits]

https://github.com/kazutakahirata commented:

Could you try to do fewer types of cleanups in one PR?  IIUC, some functions are being removed.  There are some migrations from `std::string` to `StringRef`.  Thanks!

https://github.com/llvm/llvm-project/pull/139332