[libc] Complete hardening of time functions and remove Y2038 limit (#203298)

Completed safety and hardening refactoring for time functions, and
removed Y2038 limitation:

* Hardening: Refactored update_from_seconds to return ErrorOr<int>
  (instead of int64_t status, as it only returns status 0 or error)
and propagated it to entrypoints. Added LIBC_CRASH_ON_NULLPTR to public
  boundaries and converted tests to death tests using SIGILL.
* Y2038: Removed the artificial int32_t max check from ctime and
ctime_r, allowing them to support timestamps beyond 2038 (up to Year
9999).
* Tests: Updated ctime/ctime_r tests to include Year 2039 test cases and
updated overflow test cases to Year 10000.
* Style: Fixed header blocks and added Doxygen comments in converter.h,
str_converter.h, and time_utils.h.
* Conventions: Fixed relative includes in converter.h and time_utils.h.

Assisted-by: Automated tooling, human reviewed.
28 files changed
tree: 37c48801b93b25520b02d5286f7768f8a749b2cd
  1. .ci/
  2. .github/
  3. bolt/
  4. clang/
  5. clang-tools-extra/
  6. cmake/
  7. compiler-rt/
  8. cross-project-tests/
  9. flang/
  10. flang-rt/
  11. libc/
  12. libclc/
  13. libcxx/
  14. libcxxabi/
  15. libsycl/
  16. libunwind/
  17. lld/
  18. lldb/
  19. llvm/
  20. llvm-libgcc/
  21. mlir/
  22. offload/
  23. openmp/
  24. orc-rt/
  25. polly/
  26. runtimes/
  27. third-party/
  28. utils/
  29. .clang-format
  30. .clang-format-ignore
  31. .clang-tidy
  32. .git-blame-ignore-revs
  33. .gitattributes
  34. .gitignore
  35. .mailmap
  36. CODE_OF_CONDUCT.md
  37. CONTRIBUTING.md
  38. LICENSE.TXT
  39. pyproject.toml
  40. README.md
  41. SECURITY.md
README.md

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