[llvm] [SandboxVec][Interval] Implement getUnionInterval() and getSingleDiff() (PR #111455)

[Alina Sbirlea via llvm-commits]

https://github.com/alinas approved this pull request.

I'm not clear if there should be another union method API returning two intervals when there is no overlap (minimal instruction coverage vs maximal).
This looks good for this purpose.

https://github.com/llvm/llvm-project/pull/111455