C++: Support models-as-data barriers and barrier guards

[MathiasVP]

Commit by commit review recommended.

• 07ac8a5 implements support for an additional parameter in barrier guards (which itself useful for barrier guards with flow states, and will be necessary to implement the MaD support)
• 4825480 is @aschackmull's WIP commit from the latest hackathon
• a11b271 finishes the implementation

@aschackmull is this something you can review?

[Copilot]

Pull request overview

This pull request implements support for models-as-data (MaD) barriers and barrier guards in C++, allowing barrier specifications to be defined through external YAML files rather than only in QL code.

Changes:

• Adds parameterized barrier guard modules to support additional parameters (kind and model identifiers) for MaD integration
• Implements interpretation of barrier and barrier guard MaD models in the flow summary and external flow infrastructure
• Adds comprehensive test coverage for both direct barriers and barrier guards with various indirection levels

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
test.ql	Adds test infrastructure for external barrier guards using the barrierNode predicate
test.ext.yml	Defines MaD models for barrier functions and barrier guard functions
test.cpp	Adds test cases demonstrating barrier guards with various indirection patterns
SsaImpl.qll	Refactors barrier guard modules to support parameterization, enabling kind/model tracking
DataFlowUtil.qll	Adds parameterized versions of BarrierGuard and InstructionBarrierGuard modules
FlowSummaryImpl.qll	Implements interpretation of barrierModel and barrierGuardModel from MaD specifications
ExternalFlow.qll	Implements the main logic for converting MaD barrier specifications to barrier nodes

Comments suppressed due to low confidence (1)

cpp/ql/lib/semmle/code/cpp/dataflow/ExternalFlow.qll:1046

• Incorrect apostrophe usage: "GuardValues" should be "GuardValues" and "Booleans" should be "Booleans" (no apostrophes for plurals).

    // converting the barrier guard API to use `GuardValue`s instead `Boolean`s

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