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

[Gábor Horváth via cfe-commits]

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@@ -631,8 +631,16 @@ This warning may produce false-positives diagnostics when it cannot fully model
   }];
 }
 
+def LifetimeSafetyInapplicableLifetimeboundReturn
+    : DiagGroup<"lifetime-safety-inapplicable-lifetimebound-return"> {
+  code Documentation = [{
+Detects uses of [[clang::lifetimebound]] that have no effect because the function return type cannot carry a lifetime.
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Xazax-hun wrote:

I guess this is not entirely true. We might have some code paths that make these types carry lifetimes locally. That being said, this diagnostic is a telltale sign that either a type misses an annotation or a lifetimebound annotation is incorrect. I wonder if the diagnostic message should be formulated like that. 

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