[llvm] [CMake] Passthrough variables for packages to subbuilds (PR #107611)

[Petr Hosek via llvm-commits]

petrhosek wrote:

> Do you happen to know which order CMake passes in the `-C` arg for `CMAKE_CACHE_DEFAULT_ARGS` vs. the arguments inside `CMAKE_ARGS`? If the `-C` goes before `CMAKE_ARGS` (which I suspect), there's a potential interaction with https://reviews.llvm.org/D158791; if you want to use that mechanism to override any of these variables, you'll have to specify `FORCE`. That's fine, just something that came to mind.

It looks like the `-C` for `CMAKE_CACHE_DEFAULT_ARGS` comes after the `CMAKE_ARGS` so this shouldn't be an issue. I'd also like to move more of `CMAKE_ARGS` to `CMAKE_CACHE_ARGS` and `CMAKE_CACHE_DEFAULT_ARGS` in follow up changes since I think it's cleaner and avoid the possibility of hitting command line length limit on Windows.

https://github.com/llvm/llvm-project/pull/107611