This revision adds the infrastructure for `Debug Actions`. This is a DEBUG only
API that allows for external entities to control various aspects of compiler
execution. This is conceptually similar to something like DebugCounters in LLVM, but at a lower level. This framework doesn't make any assumptions about how the higher level driver is controlling the execution, it merely provides a framework for connecting the two together. This means that on top of DebugCounter functionality, we could also provide more interesting drivers such as interactive execution. A high level overview of the workflow surrounding debug actions is
shown below:
* Compiler developer defines an `action` that is taken by the a pass,
transformation, utility that they are developing.
* Depending on the needs, the developer dispatches various queries, pertaining
to this action, to an `action manager` that will provide an answer as to
what behavior the action should do.
* An external entity registers an `action handler` with the action manager,
and provides the logic to resolve queries on actions.
The exact definition of an `external entity` is left opaque, to allow for more
interesting handlers.
This framework was proposed here: https://llvm.discourse.group/t/rfc-debug-actions-in-mlir-debug-counters-for-the-modern-world
Differential Revision: https://reviews.llvm.org/D84986
River Riddle