[clang] Reapply "[Clang][CWG1815] Support lifetime extension of temporary created by aggregate initialization using a default member initializer" (PR #97308)

[via cfe-commits]

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@@ -5479,6 +5479,7 @@ void Sema::InstantiateVariableInitializer(
         *this, Sema::ExpressionEvaluationContext::PotentiallyEvaluated, Var);
 
     keepInLifetimeExtendingContext();
+    keepInRebuildDefaultArgOrInitContext();
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cor3ntin wrote:

I am not sure that setting the flag in a one line function is better than doing it directly there

https://github.com/llvm/llvm-project/pull/97308