[libc] [llvm] [libc] Refactor `BigInt` (PR #86137)

[Nick Desaulniers via llvm-commits]

https://github.com/nickdesaulniers approved this pull request.

Please revisit some of the earlier comments and either mark them resolved, update the code to address them, or provide some form of feedback on the suggestions.

https://github.com/llvm/llvm-project/pull/86137