[mlir][CAPI] Include anchor op in mlirParsePassPipeline

The pipeline string must now include the pass manager's anchor op. This
makes the parse API properly roundtrip the printed form of a pass
manager. Since this is already an API break, I also added an extra
callback argument which is used for reporting errors.

The old functionality of appending to an existing pass manager is
available through `mlirOpPassManagerAddPipeline`.

Reviewed By: mehdi_amini, ftynse

Differential Revision: https://reviews.llvm.org/D136403