Add an MLIR example of some mimimal example of MLIR setup

These may serve as example and showcase of the MLIR binary footprint.

Right now a release build of these tools on a linux machine shows:

- mlir-cat: 2MB
  This includes the Core IR, the textual parser/printer, the support for
  bytecode.
- mlir-minimal-opt: 3MB
  This adds all the tooling for an mlir-opt tool: the pass infrastructure
  and all the instrumentation associated with it.
- mlir-miminal-opt-canonicalize: 4.8MB
  This add the canonicalizer pass, which pulls in all the pattern/rewrite
  machinery, including the PDL compiler and intepreter.

Differential Revision: https://reviews.llvm.org/D156218