[clang] [Clang] Fix the normalization of fold constraints (PR #177531)

[Corentin Jabot via cfe-commits]

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@@ -2000,14 +2009,20 @@ class SubstituteParameterMappings {
   const MultiLevelTemplateArgumentList *MLTAL;
   const ASTTemplateArgumentListInfo *ArgsAsWritten;
 
-  bool InFoldExpr;
+  // When normalizing a fold constraint, e.g.
+  //   C<Pack1, Pack2...> && ...
+  // we want the TreeTransform to expand only Pack2 but not Pack1,
+  // since Pack1 will be expanded during the evaluation of C.
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cor3ntin wrote:

```suggestion
  // since Pack1 will be expanded during the evaluation of the fold expression.
```

https://github.com/llvm/llvm-project/pull/177531