[mlir][python] Provide some methods and properties for API completeness

When writing the user-facing documentation, I noticed several inconsistencies
and asymmetries in the Python API we provide. Fix them by adding:

- the `owner` property to regions, similarly to blocks;
- the `isinstance` method to any class derived from `PyConcreteAttr`,
  `PyConcreteValue` and `PyConreteAffineExpr`, similar to `PyConcreteType` to
  enable `isa`-like calls without having to handle exceptions;
- a mechanism to create the first block in the region as we could only create
  blocks relative to other blocks, with is impossible in an empty region.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D111556