[Clang] [C2y] Implement N3355 ‘Named Loops’ (#152870)

This implements support for [named
loops](https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3355.htm) for
C2y. 

When parsing a `LabelStmt`, we create the `LabeDecl` early before we parse 
the substatement; this label is then passed down to `ParseWhileStatement()` 
and friends, which then store it in the loop’s (or switch statement’s) `Scope`; 
when we encounter a `break/continue` statement, we perform a lookup for 
the label (and error if it doesn’t exist), and then walk the scope stack and 
check if there is a scope whose preceding label is the target label, which 
identifies the jump target.

The feature is only supported in C2y mode, though a cc1-only option
exists for testing (`-fnamed-loops`), which is mostly intended to try
and make sure that we don’t have to refactor this entire implementation
when/if we start supporting it in C++.

---------

Co-authored-by: Balazs Benics <benicsbalazs@gmail.com>
49 files changed
tree: 5ed6131d9d0abadc4e4726509c3c01cdf264600d
  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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