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

[Youngsuk Kim via llvm-commits]

JOE1994 wrote:

> 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?

AFAIK, there is no `raw_string_ostream::c_str` function.
The `.c_str()` calls in the diff are calls to the [`std::string::c_str`](https://cplusplus.com/reference/string/string/c_str/) function.

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