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

[Yuriy Chernyshov via cfe-commits]

georgthegreat wrote:

We have pretty large codebase and we use custom command-graph based build system (consider bazel as a well-known example of such buld system). The system uses module as a basic unit, modules might depend one each other, dependencies might affect dependant modules (but not vise versa).

In order to reduce the total size of the command graph, we would like to reduce the amount of `-I` flags induced by the most common modules.

libunwind is one of such modules: almost every module transitively depend on it yet very little do include `unwind.h` directly.

We include unwind.h using a repository-root-relative-path (i. e. `contrib/libs/libunwind/include/unwind.h`).
Everything else just works if we apply this patch.

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