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

[via cfe-commits]

================
@@ -750,8 +750,33 @@ void InitListChecker::FillInEmptyInitForField(unsigned Init, FieldDecl *Field,
     if (Field->hasInClassInitializer()) {
       if (VerifyOnly)
         return;
-
-      ExprResult DIE = SemaRef.BuildCXXDefaultInitExpr(Loc, Field);
+      ExprResult DIE;
+      {
+        // Enter a lifetime extension context, then we can support lifetime
+        // extension of temporary created by aggregate initialization using a
+        // default member initializer (DR1815 https://wg21.link/CWG1815).
+        //
+        // In a lifetime extension context, BuildCXXDefaultInitExpr will clone
+        // the initializer expression on each use and the temporaries which
+        // actually bound to a reference member should be extended here.
----------------
cor3ntin wrote:

```suggestion
        // binds to a reference member should be extended here.
```

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