[llvm] Export `LLVM_VERSION_*` CMake variables to `PARENT_SCOPE` (PR #83346)

[Mehdi Amini 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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joker-eph wrote:

This seems a bit ad-hoc compared to every else that could be exported?

You mention parity with FindPackage(LLVM) in CMake: what else does  FindPackage exports?

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