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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

Don't we already have a standalone build for MLIR? You could theoretically add it as an external project in the clang build itself, but I'm not sure if that's desirable. Dependency management goes out  the window once you have a project barrier.

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