[llvm] [mlir] [mlir][core] Add an MLIR "pattern catalog" generator (PR #146228)

[Jeremy Kun via llvm-commits]

j2kun wrote:

Is not relying on a filesystem all that critical? The mutex is mildly annoying, but it's scoped pretty tightly, and the alternative requires changes in a few other components (wouldn't I also need to plumb these environment variables through the lit config?) and additional cleanup of the output.

I agree setting environment variables would be the cleanest approach. I'm just not confident in my ability to push a big change to `cl::opt` through, and using the filesystem seems good enough.

https://github.com/llvm/llvm-project/pull/146228