[Clang] FunctionEffects: Make a separate diagnostic group for redeclarations/overrides where effects are implicit. (#148690)

The current function effect diagnostics include these behaviors:

When you declare a function `nonblocking` (typically in a header) and
then omit the attribute on the implementation (or any other
redeclaration), Clang warns: attribute 'nonblocking' on function does
not match previous declaration.

But if a `nonblocking` function is a C++ virtual method, then overrides
are implicitly nonblocking; the attribute doesn't need to be explicitly
stated.

These behaviors are arguably inconsistent -- and also, both, more
pedantic than the rest of the function effect diagnostics.

This PR accomplishes two things:

- Separates the diagnostic on a redeclaration into a new group,
`-Wfunction-effect-redeclarations`, so it can be disabled independently.

- Adds a second diagnostic to this new group, for the case of an
override method missing the attribute. (This helps in a situation where
I'm trying to add `nonblocking` via a macro that does other things and I
want to know that the macro is missing on an override declaration.)

---------

Co-authored-by: Doug Wyatt <dwyatt@apple.com>
Co-authored-by: Sirraide <aeternalmail@gmail.com>
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tree: 7e5c9bfbd608df58c5a1caf2f4a39db5ea249ed4
  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. runtimes/
  25. third-party/
  26. utils/
  27. .clang-format
  28. .clang-format-ignore
  29. .clang-tidy
  30. .git-blame-ignore-revs
  31. .gitattributes
  32. .gitignore
  33. .mailmap
  34. CODE_OF_CONDUCT.md
  35. CONTRIBUTING.md
  36. LICENSE.TXT
  37. pyproject.toml
  38. README.md
  39. SECURITY.md
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