[llvm] Export `LLVM_VERSION_MAJOR` CMake variable to `PARENT_SCOPE` (PR #83346)

[Benoit Jacob via llvm-commits]

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@@ -28,6 +28,15 @@ if(NOT DEFINED LLVM_VERSION_SUFFIX)
   set(LLVM_VERSION_SUFFIX git)
 endif()
 
+# Export a few LLVM version identifiers for users who use LLVM as a subdir.
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bjacob wrote:

Done.

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