Export `LLVM_VERSION_MAJOR` CMake variable as a directory property (#83346)

This allows users who include `llvm-project` as a subrepository to
access the `LLVM_VERSION_MAJOR` variable on par with if they were
relying on an installed LLVM and `FindLLVM.cmake`. They just need to do
something like:

```
  get_directory_property(LLVM_VERSION_MAJOR DIRECTORY "third_party/llvm-project/llvm" LLVM_VERSION_MAJOR)
```

Context: https://github.com/openxla/iree/pull/16606 -- like other
projects with similar needs that I found by some googling, our
work-around had been to rely on the CMake cached variable
`CLANG_EXECUTABLE_VERSION`. Being cached, it over time inevitably ended
up having a wrong value.