Switch the remaining C++17 components to C++20

[achirkin]

Update to C++20 CMake components that are still C++17:

• C tests
• benchmarks

Also add a small workaround for a known nvcc issue.

Supersedes #1795 and #1796

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[achirkin]

/ok to test

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR upgrades C/CUDA language standards from 17 to 20 across test and benchmark CMake configurations, refactors test parameterization in an ANN test suite to use direct Values(...) instead of a named vector, and adjusts JSON diagnostics macro ordering in a benchmark source file.

Changes

C++20 Language Standard Upgrade

Layer / File(s)	Summary
CMake Configuration c/tests/CMakeLists.txt, cpp/bench/ann/CMakeLists.txt	CXX_STANDARD and CUDA_STANDARD are upgraded from 17 to 20 with CXX_STANDARD_REQUIRED and CUDA_STANDARD_REQUIRED set to ON for test executables and ANN benchmark targets.

Test Suite and Benchmark Refinements

Layer / File(s)	Summary
Test Parameterization c/tests/neighbors/ann_mg_c.cu	INSTANTIATE_TEST_SUITE_P is refactored to use ::testing::Values(...) directly instead of building and passing a named test_inputs vector, removing indirection while preserving parameter coverage for IVF-Flat, IVF-PQ, and CAGRA algorithms.
Benchmark Compilation cpp/bench/ann/src/diskann/diskann_benchmark.cpp	JSON_DIAGNOSTICS macro definition is positioned before the nlohmann/json.hpp include to ensure compile-time JSON library configuration.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'Switch the remaining C++17 components to C++20' accurately reflects the main change of updating CMake components from C++17 to C++20 across C tests and benchmarks.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Description check	✅ Passed	The PR description clearly describes the changeset: updating C++17 components to C++20 for C tests and benchmarks, plus adding an nvcc workaround.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[achirkin]

/merge