[clang] "modular_format" attribute for functions using format strings (#147431)

This provides a C language `modular_format` attribute. This combines
with information from the existing `format` to set the new IR
`modular-format` attribute.

The purpose of these attributes is to enable "modular printf". A
statically linked libc can provide a modular variant of printf that only
weakly references implementation routines. The link-time symbol `printf`
would strongly reference aspect symbols (e.g. for float, fixed point,
etc.) that are provided by those routines, restoring the status quo.
However, the compiler could transform calls with constant format strings
to calls to the modular printf instead, and at the same time, it would
emit strong references to the aspect symbols that are needed to
implement the format string. Then, the printf implementation would
contain only the union of the aspects requested.

See issue #146159 for context.
11 files changed
tree: 6162cc9cc90ef9b95ce3426ea4a5e03f6321efe5
  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
README.md

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