[libunwind] Add a _LIBUNWIND_VERSION macro

This allows us to detect whether we're being compiled with LLVM's libunwind
more easily, without CMake having to set explicit variables.

As discussed in https://llvm.org/D119538.

Differential Revision: https://reviews.llvm.org/D121015

GitOrigin-RevId: f29002a4b71b7734dfc4bd0e62eec2d341edb3fd
2 files changed