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

[Maksim Levental via llvm-commits]

makslevental wrote:

> I had originally tried something like this, and my obstacle was that I couldn't get the test suite to produce useful outputs because stuff was piped to FileCheck (when I tried just dumping to llvm::outs()), and I didn't have a way to modify the RUN command of all the lit tests in a systematic/pragmatic way to add a debug-only flag.

If you make [`FileCheck -dump-input`](https://github.com/llvm/llvm-project/blob/caf26b9437ae3e26d4c9c8680d3e755aafeda6ed/llvm/utils/FileCheck/FileCheck.cpp#L127) "settable" via env var (which shouldn't be too controversial?) then I think it would work (although you'd get the IR output too). Still I think that's better than relying on a filesystem and etc.

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