[flang][OpenMP] Clarify confusing error message (#144707)

The message "The atomic variable x should occur exactly once among the
arguments of the top-level [...] operator" was intended to convey that
(1) an atomic variable should be an argument, and (2) it should be
exactly one of the arguments. However, the wording turned out to be
sowing confusion instead.

Rework the corresponding check, and emit an individual error message for
each problematic situation:
- "atomic variable cannot be a proper subexpression of an argument",
- "atomic variable should appear as an argument",
- "atomic variable should be exactly one of the arguments".

Fixes https://github.com/llvm/llvm-project/issues/144599
4 files changed
tree: 066a75702fcae350bf17c7fe07bba80d040638cd
  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. libunwind/
  16. lld/
  17. lldb/
  18. llvm/
  19. llvm-libgcc/
  20. mlir/
  21. offload/
  22. openmp/
  23. polly/
  24. pstl/
  25. runtimes/
  26. third-party/
  27. utils/
  28. .clang-format
  29. .clang-format-ignore
  30. .clang-tidy
  31. .git-blame-ignore-revs
  32. .gitattributes
  33. .gitignore
  34. .mailmap
  35. CODE_OF_CONDUCT.md
  36. CONTRIBUTING.md
  37. LICENSE.TXT
  38. pyproject.toml
  39. README.md
  40. SECURITY.md
README.md

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