[llvm] Avoid exposing unknown git repositories (PR #105220)

[Petr Hosek via llvm-commits]

petrhosek wrote:

While I understand the concern, I'm a bit concerned that this could be undesirable to many vendors and there should be an RFC to raise the awareness and get more feedback. If we decide to go forward with this change, I also think there should be a CMake option controlling this behavior rather than relying on `LLVM_FORCE_VC_REPOSITORY`.

https://github.com/llvm/llvm-project/pull/105220