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

[Fangrui Song via llvm-commits]

MaskRay wrote:

@aengelke 

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()`.

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