[clang] [LifetimeSafety] Warn on inapplicable `[[clang::lifetimebound]]` when return type can not carry a lifetime (PR #203767)

[Gábor Horváth via cfe-commits]

https://github.com/Xazax-hun commented:

I think with the correct rephrasing of the diagnostics this change looks good to me as an optional warning. Could be very useful to help adopt annotations. 

https://github.com/llvm/llvm-project/pull/203767