[clang] [clang] support Wold-style-declaration (PR #78837)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman commented:

Thank you for working on this! Please be sure to add release notes to clang/docs/ReleaseNotes.rst so users know about the new diagnostic capabilities.

https://github.com/llvm/llvm-project/pull/78837