[clang] [Clang] Fix handling of brace ellison when building deduction guides (PR #94889)

[Haojian Wu via cfe-commits]

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@@ -305,14 +305,32 @@ namespace {
 /// structured list even in 'verify only' mode, so that we can track which
 /// elements need 'empty' initializtion.
 class InitListChecker {
+public:
+  struct CandidateParamTypesForAggregateDeduction {
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hokein wrote:

nit: instead of using two pointers pointing to the external storage, I'd suggest storing value in this struct, and use a pointer of `CandidateParamTypesForAggregateDeduction` in the `InitListChecker`.

https://github.com/llvm/llvm-project/pull/94889