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

[Jeremy Kun via llvm-commits]

j2kun wrote:

> I support this with every fiber of my being! Bravo. 
> 
> My only comment (aside from `getDebugName`) is that it shouldn't mess with the filesystem and it should just dump to the screen. If you put the impl into it's own cpp file and then do `
> #define DEBUG_TYPE "pattern-catalog"`, then people will be able to produce the catalog by just doing `debug-only=pattern-catalog` and piping (eg to a file on disk or whatever).

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.

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