Unified: Fix AST, CFG, and data flow rules for assignments

[asgerf]

Performs some cross-cutting changes to assignments, ultimately improving data flow:

• Assignments are now mapped to BinaryExpr in the raw AST schema.
• The QL classes AssignExpr and CompoundAssignExpr emulate the original AST nodes on the QL side.
• Updates CFG to support assignments. This was previously blocked on assignments not being subtypes of BinaryExpr.
• Updates data flow to associate more precise CFG nodes with assignments.
• Lastly, associates post-update nodes with a more precise CFG node. This required a minor update to the SSA library, hence the many DCA runs. Other languages should not be affected yet, but they can use the same trick when they are ready.

The last point is perhaps worth elaborating on. Most languages have a known issue with false flow in this case

x.f = foo(x)

where the post-update for x on the left-hand side flows into the x on the right-hand side. This fixes the issue by inserting a synthetic read of x after the assignment and essentially using that as the post-update node.

This does require a bit of follow-up work in case x is re-assigned inside the RHS expression, but since Swift assignments are not expressions, this can't happen in Swift, and we can't even write a test case for it. But the fix would be to block the post-update node if the two reads don't have the same SSA definition.

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[asgerf]

Rerun has been triggered: 7 restarted 🚀

• Swift Integration tests MacOS
• CPP Compile Queries
• CLI Integration tests MacOS
• Ruby Integration tests Windows
• C# Integration tests Windows
• Python Integration tests MacOS
• Python Integration tests Linux