[clang] Reapply "[Clang][CWG1815] Support lifetime extension of temporary created by aggregate initialization using a default member initializer" (PR #108039)

[via cfe-commits]

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@@ -7810,9 +7813,11 @@ class Sema final : public SemaBase {
   }
 
   bool isInLifetimeExtendingContext() const {
-    assert(!ExprEvalContexts.empty() &&
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yronglin wrote:

Oh, I understand your concern. It's because we use `Sema::currentEvaluationContext()`, it's already has checked that `ExprEvalContexts` is not empty, so I removed assertion in `isInLifetimeExtendingContext`.
```C++
  const ExpressionEvaluationContextRecord &currentEvaluationContext() const {
    assert(!ExprEvalContexts.empty() &&
           "Must be in an expression evaluation context");
    return ExprEvalContexts.back();
  };
```

https://github.com/llvm/llvm-project/pull/108039