[llvm] [XCOFF][OBJECT] get symbol size by calling XCOFF interfaces (PR #67304)

[James Henderson via llvm-commits]

https://github.com/jh7370 commented:

Basically looks good from my point of view. However, I'd like @diggerlin to give an LGTM, and I'd also like to see it in its final form once #67284 has landed. In other words, once @diggerlin is happy and the other PR has landed, please rebase this PR and force push, making sure that the PR description lines up with your final intended commit message, so that I can review that too.

https://github.com/llvm/llvm-project/pull/67304