[clang] [C] Update the -Wdefault-const-init-unsafe wording (PR #138266)

[Erich Keane via cfe-commits]

https://github.com/erichkeane approved this pull request.

I'm not sure I buy the justification, and the flag-diag there is perhaps 'cute' (and frankly, if we find ourself doing that again, I would vastly prefer we do something like this in tablegen/Diagnostics directly to auto generate/check these things in a way that cannot be called.

https://github.com/llvm/llvm-project/pull/138266