[llvm] [llvm][CodeGen] Avoid 'raw_string_ostream::str' (NFC) (PR #97318)

[Youngsuk Kim via llvm-commits]

JOE1994 wrote:

> I think `str()` may actually be desirable so that raw_ostream.tell() and `str.size()` can be different. There is a performance opportunity. See #97704
> 
> Please hold off migrating `str()`.

I'll pause with the `.str()` migration effort. Thank you for the heads up.

https://github.com/llvm/llvm-project/pull/97318