[clang] [Clang] Remove the PackExpansion restrictions for rewrite substitution (PR #126206)

[Haojian Wu via cfe-commits]

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@@ -2041,9 +2041,11 @@ TemplateName TemplateInstantiator::TransformTemplateName(
         // We're rewriting the template parameter as a reference to another
         // template parameter.
         if (Arg.getKind() == TemplateArgument::Pack) {
-          assert(Arg.pack_size() == 1 && Arg.pack_begin()->isPackExpansion() &&
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hokein wrote:

nit: this code is duplicated 3 times, I think we can pull out a common function for it.

https://github.com/llvm/llvm-project/pull/126206