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

[Tamir Duberstein via cfe-commits]

tamird wrote:

> I don't quite understand why the fix needs to be this complicated. From the bug report, this is just a hack you did on Linux that you want to have something similar on macOS. I don't think this is worth changing the default behavior of the macOS build is.
>
> [...]
> 
> The simple fix for your CI of bpf-linker can be as simple as create a wrapper around the linker to only set DYLD_LIBRARY_PATH for your tool.

I think you're suggesting a shorter path to a hack that might solve our proximate problem.

This PR is something else: it is attempting to fix the seemingly incorrect default behavior on macOS which has diverged from other unixen for reasons that were never written down.

> macOS has a two level namespace that this can introduce some undesired behavior, like DYLD_LIBRARY_PATH can't be used to select dylib built with different versions

That's the whole point: why would you want to pick an unspecified version? IIUC that behavior doesn't work today on Linux and for good reason.

> and this will also allow library of different version to co-exist in the same process, which is really bad for LLVM libraries.

I'm not sure how that would happen, but I'm not sure it's relevant because the goal is to make behavior more consistent across platforms.

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