[compiler-rt] [llvm] [ORC][MachO] Use a per-graph compact-unwind dso_base (PR #208931)

[Lang Hames via llvm-commits]

lhames wrote:

> Hi @lhames, would you mind helping review this PR when you have a chance? I wonder if it is a sound patch to add. Thanks for attention to this:)

The standalone `UnwindInfoRegistrationPlugin` already builds a local header, so individual objects can be linked at arbitrary locations. `MachOPlatform` was written assuming slab allocation (though it's poorly documented, and not enforced) -- I'd never considered the possibility that anyone would want MachOPlatform without slab allocation.

Do you just need unwind info for your use case, or do you use other MachOPlatform features? And could you reasonably use the slab allocator for your use case?

https://github.com/llvm/llvm-project/pull/208931