This changes the flow to be taint rather than data flow, and it extends
it to include chi instructions with unknown type as long as they're not
for the `AliasedVirtualVariable`.
We're losing three good test results because these tests are not
affected by `DefaultTaintTracking.qll`. The taint step added here can
later be ported to `TaintTrackingUtil.qll` to recover these results, but
we probably want a better API than transitive-closure search through
instructions before doing that.
We designed the IR's `DataFlow::Node.asExpr` very carefully so that it's
suitable for taint tracking, but then we didn't use it in
`DefaultTaintTracking.qll`. This meant that the sources in
`ArithmeticWithExtremeValues.ql` didn't get associated with any
`Instruction` and thus didn't propagate anywhere.
With this commit, the mapping of `Expr`-based sources to IR data-flow
nodes uses `asExpr`.
There was already a `WriteSideEffectInstruction` class that served as a
superclass for all the specific write side effects. This new class
serves the same purpose for read side effects.
When building SSA, we'll be assuming that stack variables do not escape, at least until we improve our alias analysis. I've added a new `IREscapeAnalysisConfiguration` class to allow the query to control this, and a new `UseSoundEscapeAnalysis.qll` module that can be imported to switch to the sound escape analysis. I've cloned the existing IR and SSA tests to have both sound and unsound versions. There were relatively few diffs in the IR dump tests, and the sanity tests still give the same results after one change described below.
Assuming that stack variables do not escape exposed an existing bug where we do not emit an `Uninitialized` instruction for the temporary variables used by `return` statements and `throw` expressions, even if the initializer is a constructor call or array initializer. I've refactored the code for handling elements that initialize a variable to share a common base class. I added a test case for returning an object initialized by constructor call, and ensured that the IR diffs for the existing `throw` test cases are correct.
