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.

llvm/CMakeLists.txt

diff --git a/llvm/CMakeLists.txt b/llvm/CMakeLists.txt
index 651f178..111c8cf 100644
--- a/llvm/CMakeLists.txt
+++ b/llvm/CMakeLists.txt
@@ -28,6 +28,8 @@
   set(LLVM_VERSION_SUFFIX git)
 endif()
 
+set_directory_properties(PROPERTIES LLVM_VERSION_MAJOR "${LLVM_VERSION_MAJOR}")
+
 if (NOT PACKAGE_VERSION)
   set(PACKAGE_VERSION
     "${LLVM_VERSION_MAJOR}.${LLVM_VERSION_MINOR}.${LLVM_VERSION_PATCH}${LLVM_VERSION_SUFFIX}")