[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.

@MaskRay
If I add a function like `raw_string_ostream::str_size()` before removing `raw_string_ostream::str()`,
the `.str()` migration effort wouldn't hinder with leveraging the performance opportunity.
Would you mind if I resume the `llvm::raw_string_ostream::str()` migration effort? 

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