Refs #28455 -- Implemented private API methods for preventing QuerySet cloning.

Multiple calls are idempotent assuming they're balanced. Also, multiple
calls to disable cloning followed by a single call to re-enable cloning
will subsequently cause clones to occur - it is not a stack, just a
toggle.

@contextlib.contextmanager is intentionally not used for performance
reasons:
- decorator takes 1.1µs to execute, or 2µs if used correctly in a
  `with ...:` statement
- custom class takes 300ns to execute, or 900ns if used correctly in a
  `with ...:` statement

Based on work originally done by Anssi Kääriäinen and Tim Graham.

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