Track dialects in passes

[rikhuijzer]

Currently, calling --convert-mlir-to-llvmir on say

func.func @main {
  arith.constant 0 : i32
  return
}

will give a weird error in XRCF, but it should actually keep track of dialects like MLIR:

$ mlir-translate --mlir-to-llvmir tmp.mlir
tmp.mlir:6:10: error: Dialect `arith' not found for custom op 'arith.constant'
    %0 = arith.constant 0 : i32

EDIT: I'm gonna let this sit for a while. I'm not sure whether dialects are the best way to handle this. The main idea of dialects is that the op prefix (like arith.constant) makes it easy to group operations, but then there is llvm.mlir.constant which means that preferably we want nested namespaces.

So essentially it's not a dialect but a namespace, so a module?