[clang] [clang] Do not substitute parameter pack while retaining the pack expansion (PR #108197)

[Ilya Biryukov via cfe-commits]

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@@ -113,9 +113,11 @@ class TreeTransform {
   class ForgetPartiallySubstitutedPackRAII {
     Derived &Self;
     TemplateArgument Old;
+    Sema::ArgumentPackSubstitutionIndexRAII ResetPackSubstIndex;
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ilya-biryukov wrote:

Could you add a short comment mentioning that we need this because many code paths assume correct index corresponds to the pack being present and do not do any extra checking?

https://github.com/llvm/llvm-project/pull/108197