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

[Youngsuk Kim via llvm-commits]

JOE1994 wrote:

CI is failing due to a LIT failure unrelated to this revision.
Will "Update with rebase" to see how it goes.

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