[clang] Clean up array allocation logic in Sema::BuildCXXNew (PR #186617)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

I _think_ I'd lean towards a std::variant approach, although std::variant's interface is annoying as well - a full abstraction class is too heavy weight. Maybe just calculating into local bools is better.

https://github.com/llvm/llvm-project/pull/186617