[llvm] [StackFrameLayoutAnalysis] Add basic Scalable stack slot output (PR #99883)

[David Green via llvm-commits]

davemgreen wrote:

The test I pre-committed for this earlier is acting differently on different bots due to the sort order of the objects with identical offsets not being deterministic. Hopefully this should fix it as the objects will have different orders now.

https://github.com/llvm/llvm-project/pull/99883