[llvm] [llvm-size] Avoid unneeded uses of 'raw_string_ostream::str' (NFC) (PR #108490)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Given that this seems to be building with the pre-merge checks, I'm probably completely missing something, so apologies for the question, but I wanted to double check the details of the `raw_string_ostream::c_str` function, only to be completely unable to find it. Would you mind pointing out where it is, please?

https://github.com/llvm/llvm-project/pull/108490