[llvm] [Runtimes] Use order-only deps for runtime external projects (PR #185664)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

@petrhosek I revised this, it should fix the issues that I see and I think it's minimally invasive. I looked into it and I do not think there's a way to omit `BUILD_ALWAYS`. CMake doesn't track any dependencies between external projects, so the only thing we can do to ensure they're coherent is to check it every time. This PR primarily just makes the builtins step no longer clobber the dependencies while maintaining the expected build order. So, we should still build the builtins first, just without the old dependency that forced a reconfigure.

I would really like this issue to be fixed and I would prefer we have something that works rather than wait an indeterminate amount of time for something I do not think is possible in the first place.

https://github.com/llvm/llvm-project/pull/185664