[libc][realpath] Implement symbolic path resolution (#204467)

This PR contains an incremental implementation of `realpath` that just
simplifies path traversals (e.g. `"/a/./../b"` -> `"/b"`). The complete
implementation would perform `getcwd`/`statx`/`readlinkat` to actually
resolve symlinks, validate paths, etc. This PR aims to add as much
functionality/error handling as possible without making any syscalls.

This implementation lives in `libc/src/stdlib/linux/`, since follow-on
PRs will rely on linux-specific syscalls. For testing purposes, I've
added the entrypoint for x86-64 linux when
`LLVM_LIBC_ENABLE_EXPERIMENTAL_ENTRYPOINTS` is enabled.

Follow-on PRs to add: path validation (e.g.
paths must exist), support for cwd-relative paths, and symlink
resolution.
17 files changed
tree: 482b6bfb27f5c0f070b18a9a8691c0bf7df23874
  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
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