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

[Erich Keane via cfe-commits]

erichkeane wrote:

> > I'm not sure I buy the justification
> 
> FWIW, this came from a post-commit review comment: [#137166 (comment)](https://github.com/llvm/llvm-project/pull/137166#discussion_r2070615023)
> 
> > , 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.
> 
> Yeah, at some point we may want to update the diagnostics engine so it can track groups directly instead of by warning, or at the very least, hide this kind of hack behind an interface on `DiagnosticsEngine`.

I'd seen that, I still am not sure I buy the justification anyway, but if others want it and you're sympathetic, I am not against it.  The Diagnostics hack is wacky/hurts me, but this is the one 'pass' :D

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