[mlir][python] Fix issue in diagnostic note initialization

[rkayaith]

Summary

Previously the elements of the notes tuple would be invalid objects when
accessed from a diagnostic handler, resulting in a segfault when used.

[rkayaith]

mlir/lib/Bindings/Python/IRCore.cpp

788

I'm not super familar with what pybind is doing here, but I believe the primary difference is that it will increment the ref count of the tuple element, which wasn't being done previously: https://github.com/pybind/pybind11/blob/59f03ee389c283cde65bd800c8f32ea690daf3fd/include/pybind11/pytypes.h#L1013

[rkayaith]

add missing CHECK-LABEL

mlir/lib/Bindings/Python/IRCore.cpp

 py::tuple PyDiagnostic::getNotes() {
   checkValid();
   if (materializedNotes)
     return *materializedNotes;
   intptr_t numNotes = mlirDiagnosticGetNumNotes(diagnostic);
   materializedNotes = py::tuple(numNotes);
   for (intptr_t i = 0; i < numNotes; ++i) {
     MlirDiagnostic noteDiag = mlirDiagnosticGetNote(diagnostic, i);
-    py::object pyNoteDiag = py::cast(PyDiagnostic(noteDiag));
-    PyTuple_SET_ITEM(materializedNotes->ptr(), i, pyNoteDiag.ptr());
+    materializedNotes.value()[i] = PyDiagnostic(noteDiag);
   }
   return *materializedNotes;
 }
 
 //------------------------------------------------------------------------------
 // PyDialect, PyDialectDescriptor, PyDialects, PyDialectRegistry
 //------------------------------------------------------------------------------
 

mlir/test/python/ir/diagnostic_handler.py

def callback(d):
     print(f"DIAGNOSTIC: notes={d.notes}")
     return True
   handler = ctx.attach_diagnostic_handler(callback)
   loc = Location.unknown(ctx)
   loc.emit_error("foobar")
   assert not handler.had_error
 
 
+# CHECK-LABEL: TEST: testDiagnosticNonEmptyNotes
+@run
+def testDiagnosticNonEmptyNotes():
+  ctx = Context()
+  def callback(d):
+    # CHECK: DIAGNOSTIC:
+    # CHECK:   message='arith.addi' op requires one result
+    # CHECK:   notes=['see current operation: "arith.addi"() : () -> ()']
+    print(f"DIAGNOSTIC:")
+    print(f"  message={d.message}")
+    print(f"  notes={list(map(str, d.notes))}")
+    return True
+  handler = ctx.attach_diagnostic_handler(callback)
+  loc = Location.unknown(ctx)
+  Operation.create('arith.addi', loc=loc).verify()
+  assert not handler.had_error
+
 # CHECK-LABEL: TEST: testDiagnosticCallbackException
 @run
 def testDiagnosticCallbackException():
   ctx = Context()
   def callback(d):
     raise ValueError("Error in handler")
   handler = ctx.attach_diagnostic_handler(callback)
   loc = Location.unknown(ctx)