[clang] [mlir] [CMake] Don't reference unexported targets from dependency-only configs (PR #222630)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I can't say I completely understand what is going on here, but if @joker-eph agrees this is the right thing to do, I'm all for it.

Does this reduce ninja edges in any way when mlir is pulled in as a dependency? 

https://github.com/llvm/llvm-project/pull/222630