[clang] [clang][Sema] Handle function parameter packs in `PackDeductionScope::addPacks` (PR #215235)

[Yanzuo Liu via cfe-commits]

================
@@ -935,6 +935,11 @@ class PackDeductionScope {
       S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
       for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
         unsigned Depth, Index;
+
+        // Function parameter packs cannot be deduced.
+        if (isa_and_nonnull<ParmVarDecl>(
+                dyn_cast<NamedDecl *>(Unexpanded[I].first)))
+          continue;
----------------
zwuis wrote:

Sorry that my previous explanation is incomplete. Here is an example where `Unexpanded[i].first` can be anything other than `NamedDecl *` of the `PointerUnion`.

```cpp
template <typename...> struct S1 {};
template <typename... Ts>
struct S2 {
  template <typename... Us>
  auto f(S1<__builtin_dedup_pack<Ts...>...>,
         S1<__builtin_dedup_pack<Us...>...>) -> S1<Us...>;
};
void g(S2<int> s) { s.f<int>(); }
```

https://github.com/llvm/llvm-project/pull/215235