Add support for expected-warning and expected-note markers in mlir-opt -verify

mode.  We even diagnose mistakes nicely (aside from the a/an vowel confusion
which isn't worth worrying about):

test/IR/invalid.mlir split at line tensorflow/mlir#399:8:34: error: 'note' diagnostic emitted when expecting a 'error'
  %x = "bar"() : () -> i32    // expected-error {{operand defined here}}
                                 ^

PiperOrigin-RevId: 214773208