[libunwind] Use relative includes to allow source-based dependencies without `-I` (PR #80443)

[Louis Dionne via cfe-commits]

https://github.com/ldionne requested changes to this pull request.

We require that the right paths be setup with `-I` or `-isystem` as a general requirement of libc++, libc++abi and libunwind. This change seems brittle to me, especially if there's no test or no official guarantee being added and documented.

Basically, this might fix things for you in the immediate term, but it's really just a band-aid and it will break again in the future. Can you please provide more context about your use case and what official change you'd like us to make (now, but also going forward in the code base). Then, if we agree this is worth pursuing, we should set up some kind of test that allows us to provide that property in a guaranteed way and not regress it.

https://github.com/llvm/llvm-project/pull/80443