[mlir] Add a new context flag for disabling/enabling multi-threading

This is useful for several reasons:
* In some situations the user can guarantee that thread-safety isn't necessary and don't want to pay the cost of synchronization, e.g., when parsing a very large module.

* For things like logging threading is not desirable as the output is not guaranteed to be in stable order.

This flag also subsumes the pass manager flag for multi-threading.

Differential Revision: https://reviews.llvm.org/D79266