Rust: Adjust SSA write node for (compound) assignments

[hvitved]

Before this PR, SSA assignment definitions like x = y would use the control-flow node of x as the point where the assignment happens. This works most of the time, except when the assignee also occurs in the RHS, e.g. x = x + 1, because we evaluate assignments left-to-right.

This PR changes the above to instead use the control-flow node for the assignment itself (which comes after x and y in the control flow graph; all expressions are post-order).

[geoffw0]

Looks plausible, fixes my issue, I'd like to see the results of the DCA run before merging...

[geoffw0]

DCA LGTM. We can ignore the RequestForgery.ql test failure, that is unrelated and has been fixed on main.

[Copilot]

Pull Request Overview

This PR adjusts SSA write node handling for assignments to use the control-flow node of the assignment expression itself rather than the control-flow node of the left-hand side variable. This change ensures proper ordering when the assignee appears in both the left-hand side and right-hand side of an assignment (e.g., x = x + 1).

Key changes:

• Modified SSA write node tracking to use assignment expression CFG nodes instead of left-hand side variable CFG nodes
• Added test case for self-assignment (x2 = x2) to verify the fix
• Updated several library functions to support the new write node structure

Reviewed Changes

Copilot reviewed 10 out of 12 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
rust/ql/test/library-tests/variables/main.rs	Added test case for self-assignment to verify SSA ordering
rust/ql/test/library-tests/variables/variables.expected	Updated expected test results with new SSA node numbering
rust/ql/test/library-tests/variables/CONSISTENCY/PathResolutionConsistency.expected	Updated line number references due to test additions
rust/ql/src/queries/unusedentities/UnusedValue.ql	Updated to use new variableWrite predicate signature
rust/ql/lib/codeql/rust/elements/internal/VariableImpl.qll	Enhanced VariableWriteAccess to track the assignment expression
rust/ql/lib/codeql/rust/dataflow/internal/SsaImpl.qll	Modified variableWrite predicate to separate write node from access node
rust/ql/lib/codeql/rust/dataflow/internal/DataFlowImpl.qll	Updated to use getWriteAccess instead of getControlFlowNode
rust/ql/lib/codeql/rust/dataflow/Ssa.qll	Enhanced WriteDefinition to properly handle assignment expressions
rust/ql/lib/codeql/rust/controlflow/internal/CfgNodes.qll	Added AssignmentExprChildMapping for proper CFG node handling
rust/ql/lib/codeql/rust/controlflow/CfgNodes.qll	Added new CFG node classes for variable accesses and assignments