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

[Scott Todd via llvm-commits]

ScottTodd 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 .

Neither of those are particularly attractive options when depending on many targets in the subproject (as we do with MLIR targets). If we were just using tools like lld from LLVM as black boxes and were rarely interacting with individual libraries, header files, etc. then they might work. What's sort of tricky in our (IREE's) use of LLVM/MLIR is that we use both the individual libraries in MLIR _and_ the linking and code generation from LLVM -- and in both opinionated and general/flexible ways.

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