[clang] [llvm] [CMake] Version Darwin dylib identities (PR #189004)

[Tamir Duberstein via cfe-commits]

tamird wrote:

> There is no macOS pre commit CI. Mainly the macos linker requires libLTO name to stay the same to perform LTO builds.

OK, and what about libclang? Does that need to stay unversioned, and if so, why? I'd like to add some comments so that this isn't so fragile going forward.

https://github.com/llvm/llvm-project/pull/189004