(do not merge) use clang builtin for compile-time sequence indexing

[tenpercent]

Proposed changes

As titled

Checklist

Please put an x into the boxes that apply. You can also fill these out after creating the PR. If you're not sure, please don't hesitate to ask.

• [ ] I have added tests relevant to the introduced functionality, and the unit tests are passing locally
• [ ] I have added the test to REGRESSION_TESTS list defined at the top of CMakeLists.txt in tests/CMakeLists.txt, IF the test takes more than 30 seconds to run.
• [x] I have added inline documentation which enables the maintainers with understanding the motivation
• [ ] I have removed the stale documentation which is no longer relevant after this pull request
• [ ] (If this change is user-facing) I have added release notes which provide the end users with a brief summary of the improvement from this pull request
• [x] I have run clang-format on all changed files
• [ ] Any dependent changes have been merged

Discussion

Local benchmarking shows that non-builtin version is faster