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

[Saleem Abdulrasool via cfe-commits]

================
@@ -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()
----------------
compnerd wrote:

IIRC, even in the standalone case, you need to identify the LLVM build and pass that in via `LLVM_DIR`, why is that not sufficient for finding this include path?

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