Reduce binary size of the Microsoft demangler

[apache-hb]

This pulls the body of llvm::ms_demangle::ArenaAllocator::allocArray and llvm::ms_demangle::ArenaAllocator::alloc into a common allocAlignedBuffer function. In my testing this reduces the overall binary size of LLVMDemangle.lib by 6kb and has a negligible impact on performance.

The addNode(std::max(AllocUnit, Size)); being pulled into the common function retains the same behaviour as before as Size will always be < AllocUnit when allocating a single object.

[github-actions]

Thank you for submitting a Pull Request (PR) to the LLVM Project!

This PR will be automatically labeled and the relevant teams will be
notified.

If you wish to, you can add reviewers by using the "Reviewers" section on this page.

If this is not working for you, it is probably because you do not have write
permissions for the repository. In which case you can instead tag reviewers by
name in a comment by using @ followed by their GitHub username.

If you have received no comments on your PR for a week, you can request a review
by "ping"ing the PR by adding a comment “Ping”. The common courtesy "ping" rate
is once a week. Please remember that you are asking for valuable time from other developers.

If you have further questions, they may be answered by the LLVM GitHub User Guide.

You can also ask questions in a comment on this PR, on the LLVM Discord or on the forums.