C++: Add dataflow through NSDMI

[MathiasVP]

This PR adds dataflow through NSDMI which was left out of #21391.

The required changes are fairly simple:

• We change Function to Declaration in a few places (for the same reason as we did in C++: Handle field initialization via NSDMI in IR generation #21391).
• We add a case in getThisType to make it possible to obtain the type for this in the IRFunctions generated from the Field.

I did two small improvements in between those changes: In eb35fa0 I did a small refactoring of the parameter classes to avoid having duplicate implementations of isSourceParameterOf, and in 78c0c7c I excluded some dataflow nodes tests since it doesn't make sense to test these particular nodes.

Fixes: #21032

[Copilot]

Pull request overview

Adds missing IR dataflow support for non-static data member initializers (NSDMI), completing the work started in #21391 by ensuring this typing and callable handling work for IRFunctions generated from fields.

Changes:

• Extend IR “this type” computation to support IRFunctions generated from Field declarations.
• Generalize a few call/parameter-related APIs from Function to Declaration to cover generated callables.
• Update C++ dataflow library-tests and expected outputs to reflect the new IR flows and adjusted node typing checks.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/SsaImplCommon.qll	Updates getThisType and its doc to support IRFunctions generated from declarations beyond member functions.
cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowPrivate.qll	Generalizes static call target plumbing from Function to Declaration.
cpp/ql/lib/semmle/code/cpp/ir/dataflow/internal/DataFlowNodes.qll	Refactors parameter-node logic (incl. this) and generalizes APIs to Declaration.
cpp/ql/test/library-tests/dataflow/fields/C.cpp	Updates inline expectations to reflect new IR flow for s2 via NSDMI.
cpp/ql/test/library-tests/dataflow/fields/ir-path-flow.expected	Updates expected IR path flow edges/nodes/subpaths to include s2 flows.
cpp/ql/test/library-tests/dataflow/dataflow-tests/test.cpp	Updates inline expectations for NSDMI test to reflect new IR flow.
cpp/ql/test/library-tests/dataflow/dataflow-tests/test-source-sink.expected	Updates expected IR source→sink flows for NSDMI scenario.
cpp/ql/test/library-tests/dataflow/dataflow-tests/type-bugs.ql	Excludes FlowSummaryNode from the “node has exactly one type” check.
cpp/ql/test/library-tests/dataflow/dataflow-tests/type-bugs.expected	Updates expected results after excluding summary nodes from the IR type-bugs query.

[jketema]

LGTM if the latest DCA is happy.

[MathiasVP]

Oh. Some unexpected consistency issues in the IR folder (I always forget that we also have dataflow consistency checks in that folder!)

I'll have a quick look.

Edit: Fixed by 5db069e. This consistency violation was already present on main, and the new code just made it more noisy. I've now fixed the underlying problem.

[MathiasVP]

DCA was uneventful. Merging!