[clang] [llvm] [mlir] [CMake] Make find_package(Clang) load MLIR when ClangIR uses a full MLIR (PR #228366)

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aokblast wrote:

Hi. I think the logic is pretty the same as what I have done in my patch plus some error message. We still do find_package(MLIR) while MLIR is specified explicitly if I understand correctly.

https://github.com/llvm/llvm-project/pull/228366