[mlir:PDLL] Add support for PDL MLIR code generation

This commits starts to plumb PDLL down into MLIR and adds an initial
PDL generator. After this commit, we will have conceptually support
end-to-end execution of PDLL. Followups will add CPP generation to
match the current DRR setup, and begin to add various end-to-end
tests to test PDLL execution.

Differential Revision: https://reviews.llvm.org/D119779