Switch the CallbackOstream wrapper in the MLIR C API to an Unbuffered stream

This delegate the control of the buffering to the user of the API. This
seems like a safer option as messages are immediately propagated to the
user, which may lead to less surprising behavior during debugging for
instance.
In terms of performance, a user can add a buffered stream on the other
side of the callback.

Differential Revision: https://reviews.llvm.org/D90726