[llvm] [CMake] Add dependent project mode for Flang's MLIR (PR #221724)

[Renato Golin via llvm-commits]

https://github.com/rengolin commented:

Sounds reasonable to me, with a small nit.

Did you build to measure number of files affected / build time / size?

https://github.com/llvm/llvm-project/pull/221724