Fix the bug which causes the SmallVector to be not so small.

PackDeductionScope::PackDeductionScope calls PackDeductionScope::addPack, which might not assign a value to PackDeductionScope::FixedNumExpansions given that getExpandedPackSize returns a nullopt.

Access to an empty std::optional via the operator* is UB, and there is a case where IsExpanded is true while FixedNumberExpansions is empty. We access the empty optional, and this value is eventually used to in a call to SmallVector::reserve, which ends up trying to reserve 137 gigs of space and crashes clangd/clang++

Author: term-est

clang/lib/Sema/SemaTemplateDeduction.cpp

@@ -831,7 +831,7 @@ class PackDeductionScope {
     if (IsPartiallyExpanded)
       PackElements += NumPartialPackArgs;
     else if (IsExpanded)
-      PackElements += *FixedNumExpansions;
+      PackElements += FixedNumExpansions.value_or(1);
 
     for (auto &Pack : Packs) {
       if (Info.PendingDeducedPacks.size() > Pack.Index)