C++: Update expected test results after frontend update

[jketema]

See the internal PR for further details.

Observe that the frontend now does less constant folding, which has an impact on cpp/constant-comparison. I've addressed this by excluding expressions from contexts where a constant is required. The programmer likely made a deliberate choice in that case and expects the comparison to be constant. DCA shows one new alert on Boost, which I have not been able to capture as a test case. QA does not show a significant change in alerts for cpp/constant-comparison with the changes made here.

[Copilot]

Pull request overview

This PR updates C++ test expectations following a frontend change that reduces constant folding behavior. The changes ensure that constant comparisons in required-constant contexts (static asserts, template arguments, constexpr initializers, and enum constants) are excluded from the cpp/constant-comparison query, as these represent deliberate programmer choices.

Key changes:

• Updated compiler version support documentation (Clang 19.1.0 → 21, GCC 15.0 → 15)
• Modified cpp/constant-comparison query to exclude comparisons in constant-required contexts
• Updated test expectations across multiple test suites to reflect reduced constant folding

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
docs/codeql/reusables/supported-versions-compilers.rst	Updated supported Clang and GCC versions
cpp/ql/test/query-tests/Likely Bugs/Arithmetic/PointlessComparison/RegressionTests.cpp	Added test cases for comparisons in constant-required contexts
cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/code2.cpp	Corrected comment about false positive (now fixed)
cpp/ql/test/query-tests/Best Practices/Unused Entities/UnusedLocals/UnusedLocals.expected	Removed false positive for v2 used as template parameter
cpp/ql/test/query-tests/Best Practices/SloppyGlobal/SloppyGlobal.expected	Updated location spans for global variables
cpp/ql/test/library-tests/templates/variables/template_variables.expected	Updated location spans for template variable declarations
cpp/ql/test/library-tests/templates/isfromtemplateinstantiation/isfromuninstantiatedtemplate.expected	Updated location spans for var_template
cpp/ql/test/library-tests/scopes/parents/parents.expected	Updated location spans for template variable var
cpp/ql/test/library-tests/ir/ir/raw_ir.expected	Reordered IR instructions for lambda field initialization
cpp/ql/test/library-tests/ir/ir/aliased_ir.expected	Reordered aliased IR instructions for lambda field initialization
cpp/ql/test/library-tests/ir/ir/PrintAST.expected	Added assignment operators and changed ValueCategory for x
cpp/ql/test/library-tests/constants/addresses/addresses.expected	Added baseline for newly detected non-constant address expressions
cpp/ql/test/library-tests/constants/addresses/addresses.cpp	Uncommented stmtexpr_int test case
cpp/ql/src/change-notes/2026-01-02-constant-comparison.md	Added change note for query update
cpp/ql/src/Likely Bugs/Arithmetic/PointlessComparison.ql	Added exclusions for constant-required contexts
cpp/ql/lib/change-notes/2026-01-02-constant-folding.md	Added change note for constant folding behavior change

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