Implement use-after-free detection in the lifetime safety analysis with two warning levels.
- Added a `LifetimeSafetyReporter` interface for reporting lifetime safety issues
- Created two warning levels:
- Definite errors (reported with `-Wexperimental-lifetime-safety-permissive`)
- Potential errors (reported with `-Wexperimental-lifetime-safety-strict`)
- Implemented a `LifetimeChecker` class that analyzes loan propagation and expired loans to detect use-after-free issues.
- Added tracking of use sites through a new `UseFact` class.
- Enhanced the `ExpireFact` to track the expressions where objects are destroyed.
- Added test cases for both definite and potential use-after-free scenarios.
The implementation now tracks pointer uses and can determine when a pointer is dereferenced after its loan has been expired, with appropriate diagnostics.
The two warning levels provide flexibility - definite errors for high-confidence issues and potential errors for cases that depend on control flow.
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