Implement python iteration over the operation/region/block hierarchy.

* Removes the half-completed prior attempt at region/block mutation in favor of new approach to ownership.
* Will re-add mutation more correctly in a follow-on.
* Eliminates the detached state on blocks and regions, simplifying the ownership hierarchy.
* Adds both iterator and index based access at each level.

Differential Revision: https://reviews.llvm.org/D87982

mlir/include/mlir-c/IR.h

@@ -91,6 +91,12 @@ int mlirContextEqual(MlirContext ctx1, MlirContext ctx2);
 /** Takes an MLIR context owned by the caller and destroys it. */
 void mlirContextDestroy(MlirContext context);
 
+/** Sets whether unregistered dialects are allowed in this context. */
+void mlirContextSetAllowUnregisteredDialects(MlirContext context, int allow);
+
+/** Returns whether the context allows unregistered dialects. */
+int mlirContextGetAllowUnregisteredDialects(MlirContext context);
+
 /*============================================================================*/
 /* Location API.                                                              */
 /*============================================================================*/