[clang] [clang] CTAD: fix the aggregate deduction guide for alias templates. (PR #90894)

[Matheus Izvekov via cfe-commits]

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@@ -261,6 +261,13 @@ AG ag = {1};
 // CHECK:   | `-BuiltinType {{.*}} 'int'
 // CHECK:   `-ParmVarDecl {{.*}} 'int'
 
+template <typename X = int>
+using BG = G<int>;
+BG bg(1.0);
+// CHECK-LABEL: Dumping <deduction guide for BG>
+// CHECK:      FunctionTemplateDecl {{.*}} implicit <deduction guide for BG>
+// CHECK: |-CXXDeductionGuideDecl {{.*}} 'auto (int) -> G<int>' aggregate
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mizvekov wrote:

Nit: Since this isn't matching the structure of the AST anyway, it would probably be a good idea to avoid the possibility of unrelated test churn.

```suggestion
// CHECK: CXXDeductionGuideDecl {{.*}} 'auto (int) -> G<int>' aggregate
```

https://github.com/llvm/llvm-project/pull/90894