C++: Use the new prototype-related extensionals in MaD

[MathiasVP]

This PR makes use of the new extensionals that were added in #21870 to fix a couple of bugs in MaD.

I discussed the best way to handle "backwards compatibility" with @jketema, and we agreed to keep the old code in production for a couple of versions to allow new databases with populated class_template_generated_from and function_template_generated_from extensionals to be produced.

The DCA run is up-to-date (and boring as expected). The force-push was just me fixing up QLDoc.

[Copilot]

Pull request overview

This PR updates the C++ MaD (Models as Data) support to leverage the newer template “prototype/origin” extensionals introduced in #21870, while retaining a fallback path for older databases whose upgrade leaves those extensionals empty.

Changes:

• Add “new vs old” implementations for resolving fully-templated class/function entities, selecting based on whether the new extensionals are populated.
• Adjust signature/type-name expansion to use the fully-templated declaring class resolution.
• Update the MaD signatures test expectations to reflect the new resolution behavior.

Show a summary per file

File	Description
cpp/ql/lib/semmle/code/cpp/dataflow/ExternalFlow.qll	Switches MaD template-resolution logic to prefer new getOriginalTemplate-based extensionals with a compatibility fallback; minor QLDoc touch-ups.
cpp/ql/test/library-tests/dataflow/taint-tests/test_mad-signatures.expected	Updates expected signature/type-name output to match the new template-resolution behavior.

Copilot's findings

• Files reviewed: 2/2 changed files
• Comments generated: 2