[mlir] expose affine map to C API

This patch provides C API for MLIR affine map.
- Implement C API for AffineMap class.
- Add Utils.h to include/mlir/CAPI/, and move the definition of the CallbackOstream to Utils.h to make sure mlirAffineMapPrint work correct.
- Add TODO for exposing the C API related to AffineExpr and mutable affine map.

Differential Revision: https://reviews.llvm.org/D87617