[llvm] Export `LLVM_VERSION_MAJOR` CMake variable as a directory property (PR #83346)

[Benoit Jacob via llvm-commits]

bjacob wrote:

> Generally the recommended guidance has been to either include LLVM as an ExternalProject or to import the CMake package.

Thanks, I didn't know about https://cmake.org/cmake/help/latest/module/ExternalProject.html . TIL. FYI @ScottTodd @stellaraccident .

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