[mlir] Expose printing functions in C API

Provide printing functions for most IR objects in C API (except Region that
does not have a `print` function, and Module that is expected to be printed as
Operation instead). The printing is based on a callback that is called with
chunks of the string representation and forwarded user-defined data.

Reviewed By: stellaraccident, Jing, mehdi_amini

Differential Revision: https://reviews.llvm.org/D85748