[clang] [Clang] Include LLVM CodeGen CMake file (PR #109601)

[Tristan Ross via cfe-commits]

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@@ -1,3 +1,7 @@
+if(EXISTS ${LLVM_MAIN_SRC_DIR}/include/llvm/CodeGen AND CLANG_BUILT_STANDALONE)
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RossComputerGuy wrote:

Ok, will do.

https://github.com/llvm/llvm-project/pull/109601