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

[Erich Keane via cfe-commits]

https://github.com/erichkeane commented:

This looks good to me, and is similar to what I did for https://github.com/llvm/llvm-project/pull/221291.

Instead of this, I wonder if we should have a `MLIR_STANDALONE_MODE` instead?  It would be a mode that does the minimum and diffentiates these, plus gives the option to compile a 'minimal' MLIR for the user side as well.

In addition, my thoughts were we should have a flag that Clang/Flang can explicitly choose the set if dialects to enable.

WDYT?

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