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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > The idea/hope is that projects downstream could ALSO take advantage of this build time opt.
> 
> I don't think this is something that should be solved at this level. The PR exists to minimize dependencies that the user did not specify they want, but are needed. If you want downstream users to have minimal builds I'd look into LLVM distribution components, which are basically just big bags of CMake targets under a common name, https://llvm.org/docs/BuildingADistribution.html

I guess I can agree to disagree here.  We have similar flags (to disable tests for example), so I still think we should have a 'start with the bare minimum flag' (a dependent project mode), but obviously am invested in this getting merged sooner rather than later :) 

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