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

[Chris B via llvm-commits]

llvm-beanz wrote:

> 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.

Yep. In the past the guidance has been for projects like that to nest as LLVM subprojects. I understand reasons why that is undesirable and welcome any contributions to make your preferred workflow better supported. I just wanted to make sure you were aware that the way you're doing things is historically fraught with peril.

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