C++: Deduplicate dataflow query results

[MathiasVP]

This PR fixes the problem of duplicated dataflow results we've seen since we switched to IR-based dataflow.

To see why we get such duplicated results, consider the following query:

import cpp
import semmle.code.cpp.dataflow.new.DataFlow

module C implements DataFlow::ConfigSig {
  predicate isSource(DataFlow::Node node) { node.asExpr().(Call).getTarget().hasName("source") }

  predicate isSink(DataFlow::Node node) {
    node.asExpr() = any(Call call | call.getTarget().hasName("sink")).getAnArgument()
  }
}

module MyDataFlow = TaintTracking::Global<C>;

from DataFlow::Node source, DataFlow::Node sink
where MyDataFlow::flow(source, sink)
select sink, ""

When running on:

int source();
void sink(long);

void test() {
  int x = source();
  sink(x); // Important: There's an int-to-long conversion here
}

we get two identical results. This is because both the dataflow node for the unconverted x and the dataflow node for the converted x map to the x expression.

This PR fixes this issue by ensuring that only the fully converted expression has a result for asExpr(). Calling asExpr() on a dataflow node gives the unconverted expression, and calling asConvertedExpr() gives the converted expression.

Commit-by-commit strongly encouraged (You can see the effects of each commit in the updated .expected files):

• The first commit introduces a new predicate that we'll use to convert an instruction to an expression.
• The second predicate naively replaces the use of Instruction::getConvertedResultExpression() with this new predicate.
• The next couple of commits fixes various predicates to implement the logic around "only the fully converted instruction should have a result for asExpr.
• Finally, the last two commits fixes a bunch of queries to use asExpr instead of asConvertedExpr, and removes manual deduplication. These workarounds are no longer needed.

DCA shows significant result deduplication 🎉. There are a couple of reported lost results, but the only ones that are genuinely lost are the two cpp/very-likely-overrunning-write on vim. This is a bit unfortunate, but I can't see a good way of bringing them back with the current semantics of unique.

I'll create a follow-up issue for this last part. I don't think it should block the PR, though.

[MathiasVP]

Note: This PR isn't green on CI because there are some internal tests that need to be updated. There's a backlink to the internal PR (for those with access) where the PR is green 🟢