Add capture of "IRUnits" as context for an MLIR Action

IRUnit is defined as:

  using IRUnit = PointerUnion<Operation *, Region *, Block *, Value>;

The tracing::Action is extended to take an ArrayRef<IRUnit> as context to
describe an Action. It is demonstrated in the "ActionLogging" observer.

Reviewed By: rriddle, Mogball

Differential Revision: https://reviews.llvm.org/D144814