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

[via cfe-commits]

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@@ -874,8 +874,11 @@ class PackDeductionScope {
       SmallVector<UnexpandedParameterPack, 2> Unexpanded;
       S.collectUnexpandedParameterPacks(Pattern, Unexpanded);
       for (unsigned I = 0, N = Unexpanded.size(); I != N; ++I) {
-        unsigned Depth, Index;
-        std::tie(Depth, Index) = getDepthAndIndex(Unexpanded[I]);
+        UnexpandedParameterPack U = Unexpanded[I];
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cor3ntin wrote:

I'd like to see a comment here (this is sufficiently confusing)

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