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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> TBH, I was/am confused by that suggestion. I don't see really how it relates to a python-language or something wanting to speed up their MLIR build?

Users have always had the flexibility to selectively build dependencies, e.g. `ninja install-clang` will not build random unrelated LLVM tools but `ninja install` will. The onus is on the end user to make use of these if they need a smaller dependency tree, components are just an LLVM approved way to 'build your own distribution'. If a python language wanted to speed up their MLIR build, then MLIR just needs to provide sufficient targets for the end user.

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