[mlir][python] Usability improvements for Python bindings
Provide a couple of quality-of-life usability improvements for Python bindings,
in particular:
* give access to the list of types for the list of op results or block
arguments, similarly to ValueRange->TypeRange,
* allow for constructing empty dictionary arrays,
* support construction of array attributes by concatenating an existing
attribute with a Python list of attributes.
All these are required for the upcoming customization of builtin and standard
ops.
Reviewed By: stellaraccident
Differential Revision: https://reviews.llvm.org/D110946