[mlir] Add Python bindings for AffineExpr

This adds the Python bindings for AffineExpr and a couple of utility functions
to the C API. AffineExpr is a top-level context-owned object and is modeled
similarly to attributes and types. It is required, e.g., to build layout maps
of the built-in memref type.

Reviewed By: mehdi_amini

Differential Revision: https://reviews.llvm.org/D94225