[llvm] Export `LLVM_VERSION_MAJOR` CMake variable to `PARENT_SCOPE` (PR #83346)

[Chris B via llvm-commits]

https://github.com/llvm-beanz commented:

Generally the recommended guidance has been to either include LLVM as an ExternalProject or to import the CMake package. I mention this purely as a caution. I don't have any philosophical objection to fixing the issues related to including LLVM as a subdirectory, and generally speaking those issues are bugs.

I have two concrete suggestions below to make the code easier to read and more robust for users.

https://github.com/llvm/llvm-project/pull/83346