C++: Add a BarrierGuard module for indirect instruction/operand nodes.

[MathiasVP]

On main we have BarrierGuards for:

• Operand/Instruction nodes for reasoning about barriers at the instruction level (using DataFlow::InstructionBarrierGuard<myPredicate/3>::getABarrierNode())
• Expr nodes for reasoning about barriers at the expression level (using DataFlow::BarrierGuard<myPredicate/3>::getABarrierNode()`)
• Indirect Expr nodes for reasoning about barriers on the pointed-to value of an expression (using DataFlow::BarrierGuard<myPredicate/3>::getAnIndirectBarrierNode())

Spot an obvious missing one? We never added a BarrierGuard for reasoning about indirect instruction/operand nodes.

Barriers on indirections are nice when working with functions that validate the data pointed to by an argument to a function call. For example:

bool isValidData(char* data);
// ...
data = getInput();
if(isValidData(data)) {
 // process data
}

currently, if you want to reason about *data (that is, the indirection of data) you need to use an Expr-based BarrierGuard as there is no module for indirect instruction/operand nodes.

This PR adds DataFlow::InstructionBarrierGuard<myPredicate/3>::getAnIndirectBarrierNode() to complete the missing API. We plan on using this predicate internally at Microsoft.

Commit-by-commit review recommended:

• d63b734 adds a skeleton predicate that's necessary to compile the test in the next commit.
• 4e3b27e adds a testcase that needs indirect barrier guards for instruction/operand nodes.
• 6f3a2c4 fills in the skeleton. This is pretty much a copy/paste of the direct case, but with node.asOperand replaced with node.asIndirectOperand. So nothing interesting going on there.
• 91992e2 accepts the test changes

I have not started a DCA run as this code isn't used by any queries yet.

[Copilot]

Copilot reviewed 1 out of 4 changed files in this pull request and generated no comments.

Files not reviewed (3)

• cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowUtil.qll: Language not supported
• cpp/ql/test/library-tests/dataflow/ir-barrier-guards/test.cpp: Language not supported
• cpp/ql/test/library-tests/dataflow/ir-barrier-guards/test.ql: Language not supported

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[jketema]

LGTM