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

[Tristan Ross via cfe-commits]

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

Ok, so the solution is to set `-DLLVM_DIR` which is done by `find_package`? We already have LLVM inside Clang's CMake search path so I am not sure why the error in the PR toplevel comment is happening.

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