[clang] [Clang] Fix pack indexing profiling (PR #139276)

[Younan Zhang via cfe-commits]

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@@ -4549,6 +4549,10 @@ class PackIndexingExpr final
 
   bool isFullySubstituted() const { return FullySubstituted; }
 
+  bool isPartiallySubstituted() const {
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zyn0217 wrote:

Please document the difference with FullySubstituted

https://github.com/llvm/llvm-project/pull/139276