[clang] Reapply "[Clang] Improve diagnostics for expansion length mismatch" (PR #121044)

[Younan Zhang via cfe-commits]

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@@ -75,7 +75,7 @@ getDepthAndIndex(UnexpandedParameterPack UPP) {
   if (const auto *TTP = dyn_cast<const TemplateTypeParmType *>(UPP.first))
     return std::make_pair(TTP->getDepth(), TTP->getIndex());
 
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zyn0217 wrote:

I think the users of this function shouldn't expect a `SubstTemplateTypeParmPackType/SubstNonTypeTemplateParmPackExpr` anyway. (See the explanation below) I'll add some asserts.

https://github.com/llvm/llvm-project/pull/121044