Apply a conservative approach by filtering out results for accesses to
captured nullable values, when there is an (implicit) call to the capturing
callable which is `null`-guarded. For example:
```
bool M(int? i, IEnumerable<int> @is)
{
if (i.HasValue)
return @is.Any(j => j == i.Value); // GOOD
return false;
}
```
- Add `Caching.qll` for controlling caching across multiple files.
- Move `isUncertainRefCall()` out of cached module in `Assignable.qll` to avoid
collapsing with CFG stage.
- Remove dependency on `AlwaysNullExpr` in `NullValue::getAnExpr()` to avoid
collapsing with CFG stage.
- Avoid caching pre-SSA library as it should only be used during the CFG construction
stage.
- Extract names of properties in a propery match, using the `exprorstmt_name` relation.
- Simplify extraction of properties by not distinguishing between top-level patterns
and nested patterns.
- Introduce `PatternExpr` to capture patterns in `is` expressions, `case` statements,
and `switch` expression arms.
- Generalize `IsTypeExpr`, `IsPatternExpr`, `IsRecursivePatternExpr`, and `IsConstantExpr`
to just `IsExpr` with a member predicate `PatternExpr getPattern()`.
- Generalize `TypeCase`, `RecursivePatternCase`, and `ConstCase` to just `CaseStmt` with
a member predicate `PatternExpr getPattern()`.
- Introduce classes `Switch` and `Case` as base classes of switch statements/expressions
and case statements/switch expression arms, respectively.
- Simplify CFG logic using the generalized classes.
- Generalize guards library to cover `switch` expressions tests.
- Generalize data flow library to cover `switch` expression assignments.
- Cache predicates in the same stage using a cached module.
- Introduce `DefUse::defUseVariableUpdate()` and use in `CallableReturns.qll`.
The updated file `csharp/ql/test/library-tests/cil/dataflow/Nullness.expected`
demonstrates why this is needed.
- Utilize CIL analysis in `Guards::nonNullValue()`.
- Analyze SSA definitions in `AlwaysNullExpr`, similar to `NonNullExpr`.
