ManagedStatic: remove from DebugCounter

Follow the pattern used in MLIR for the cl::opt instances.

v2:
- make DebugCounter::isCountingEnabled public so that the
  DebugCounterOwner doesn't have to be a nested class. This simplifies
  later changes

v3:
- remove the indirection via DebugCounterOwner::instance()

Differential Revision: https://reviews.llvm.org/D129116