fix: support new clang-dumper template arguments

[lm-sousa]

The future clang-dumper emits four template-argument kinds that Clava currently rejects.

This adds parser and AST support for Declaration, NullPtr, TemplateExpansion, and StructuralValue. The structural-value model keeps the serialized type; the current dumper does not serialize the value payload.

Verification:

• gradle -p ClangAstParser compileJava
• parsed the post-release template_expansion_pack.cpp corpus with a local build of clang-dumper and reparsed Clava's generated output

Model: GPT-5.6-Luna

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[sonarqubecloud]

Quality Gate failed

Failed conditions
C Security Rating on New Code (required ≥ A)
C Reliability Rating on New Code (required ≥ A)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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