[flang][OpenMP] Add structure checks for DECLARE VARIANT (#198799)

This PR adds declare-variant structure checking. Following checks are
added:

- Validate [base:]variant arguments (including implicit base for
single-name form).
- Require exactly one MATCH clause; reject a second MATCH on the same
directive.
- Reject duplicate (base, variant) across multiple declare variant
directives.
- Reject clauses not allowed on declare variant.
- Apply shared context-selector checks to MATCH (reuse metadirective
logic).
- Require constant user conditions in MATCH for declare variant (dynamic
selectors deferred).

Refactor metadirective support:

- Extract CheckContextSelectorSpecification for reuse.
- Reject SCORE on trait sets that do not allow it (also affects
metadirective).

Co-authored-by: Cursor <cursoragent@cursor.com>
5 files changed
tree: afd8312c8b474730d333073edc2a211ff12c028d
  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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