[clang] [clang][Sema] Refine unused-member-function diagnostic message for constructors (PR #84515)

[Takuya Shimizu via cfe-commits]

hazohelet wrote:

Please add a release note line to `clang/docs/ReleaseNotes.rst` because this patch changes the diagnostic behavior of clang

https://github.com/llvm/llvm-project/pull/84515