[mlir][python] Better support for variadic regions in Python bindings

Improve support for variadic regions in ODS-generated operation view classes.
In particular, make generated constructors take an extra argument that
specifies the number of variadic regions if the operation has them. Previously,
there was no mechanism to specify a non-zero number of variadic regions. Also
generate named accessors to regions.

Reviewed By: gysit

Differential Revision: https://reviews.llvm.org/D111783