Export of internal changes

[kyessenov]

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297443901:

Use absl::Status.
Add status_macros.h for the explicit status macros import.
Make `CelExpressionFlatImpl` non-copyable to avoid a potential issue with its vector of unique pointers.

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297297141:

Temporarily disable printing of test values

296507025:

Note that interpreter tracing behaves differently when the constant folding is
enabled.
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296273889:

Add end to end tests for unknown functions and attributes. Add benchmarks (mostly characterizing worst-case merge performance).

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296273780:

Add support for unknown function results.
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296269903:

Add option for unknown function results. Final implementation in next change.
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296236495:

Add support for nested comprehensions. Before this, reusing the accumulator or iter var in a nested comprehension could put the evaluation frame in a bad state and cause a status.
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295763189:

Update references to unknownattribute set that should refer to an unknown set.
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295056234:

Add UnknownSet type -- this just coordinates updating the underlying attribute set and function results set.
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295026542:

Add UnknownFunctionResult and UnknownFunctionResultSet types.
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293733801:

absl::GetWeekday() no longer requires an absl::CivilDay argument.
Any absl::Civil* variety will do ... they all have a weekday.

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293705273:

Adding test that verify behavior of CEL ternary operator for different cases (true/false/error/unknown)
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292626176:

Fix ConstantFoldingTransform bug.
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292565730:

Separate out AttributeTrail and UnknownsUtility into their own build targets.
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291997941:

Adding Unknowns support to CEL C++ Evaluator
- Adding unknown_patterns to Activation
- Apply changes to evaluator core to support unknowns:
  - attributes stored in value stack along with corresponding values;
  - patterns stored in execution frame
  - Unknowns support in ExecutionStep implementations.
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291392455:

CEL C++ interpreter: add the missing int->string conversion function.
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287599493:

Add option for opting out of treating warnings as fatal errors.
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286093278:

Expose evaluation state from Expression parsing.

CelExpression can now initialize and use an opaque "CelEvaluationState".  This type holds all the mutable state used by CelExpression during the Trace and Evaluate calls.  Instead of allocating and initializing this state each time Evalue or Trace is called, the user can elect to Initialize a CelEvaluationState prior to those calls and pass it into the newly provided overrides.  This allows the user to remove these allocations when the CelExpression is called frequently under certain conditions (performance, realtime).
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286061190:

Remove active allocation paths from ExecutionFrame IterVars.

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286039529:

Remove active allocation paths from ValueStack.

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285120619:

Remove unused absl::string_view variables.

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285016935:

Add method for getting build warnings associated with a built expression.
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284968502:

Fix the naming  for AddResolvableEnum/RemoveResolvableEnum methods in CelExpressionBuilder
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284848907:

Demote function resolution errors at evaluation time to a warning. Client must opt out of treating warning as a fatal error (maintains current behavior).
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284594209:

Removed direct access to ExecutionFrame iteration variable map.

PiperOrigin-RevId: 297443901

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[kyessenov]

CLA manually confirmed.