[Clang][C2y] Add support for if declarations (N3356 paper) (#198244)

Add support for _if declarations_ in `C2y` mode by exploiting existing
C++ code.

The only new changes to the code is narrowing the code path to `C` mode
and introduce new errors and warnings for some pitfalls with the syntax
and what is expected from the standard and old language modes.

It should be noted that the first clause in the standard paper can only
be declaration. This means we can't allow expression statement in the
first clause of the condition, and that's the goal of the new
warnings/errors. Also this is different from C++ grammar.
9 files changed
tree: a0c7a2047c5a79ecaca9387d4d360fd99d17693b
  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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