Fix code related to some recent changes in LLVM

[boomanaiden154]

As of current TOT LLVM (past LLVM 15), the API for LLVM::Optional has changed significantly, and LLVM::None has been deprecated and many pieces have been switched over to use std::optional. This patch fixes the compilation of autoFDO in relation to these new changes.

[boomanaiden154]

Looking at some other pieces of the repository, I'm guessing these changes should be feature gated through the preprocessor to enable compatibility with multiple LLVM versions?

[shenhanc78]

Thanks Aiden. Yes, we have to support ToT as well as older LLVM source trees. However, adding Preprocessors to gate the changes seem to me a little bit cumbersome at least for changes in this PR. If it is just warnings,let's live with it for a while. What do you think?

[boomanaiden154]

It isn't just warnings. The APIs for LLVM::Optional and std::optional are not compatible, so there need to be some code changes made between the two versions. LLVM::None has also been completely removed in ToT LLVM.

[shenhanc78]

Thanks Aiden. I see. I'll merge your PR#155. (And I don't think guarding the changes is sustainable, so let's just make it buildable to ToT llvm.)