[MLIR] Factor pass timing out into a dedicated timing manager

This factors out the pass timing code into a separate `TimingManager`
that can be plugged into the `PassManager` from the outside. Users are
able to provide their own implementation of this manager, and use it to
time additional code paths outside of the pass manager. Also allows for
multiple `PassManager`s to run and contribute to a single timing report.

More specifically, moves most of the existing infrastructure in
`Pass/PassTiming.cpp` into a new `Support/Timing.cpp` file and adds a
public interface in `Support/Timing.h`. The `PassTiming` instrumentation
becomes a wrapper around the new timing infrastructure which adapts the
instrumentation callbacks to the new timers.

Reviewed By: rriddle, lattner

Differential Revision: https://reviews.llvm.org/D100647