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

[Saleem Abdulrasool 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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compnerd wrote:

I think that I would rather have the cases mirrored here - the path should always exist in general, it is only the standalone case that is special.

```suggestion
if(CLANG_BUILT_STANDALONE AND EXISTS ${LLVM_MAIN_SRC_DIR}/include/llvm/CodeGen)
```

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