[clang] [clang-tools-extra] [Clang] Add a builtin that deduplicate types into a pack (PR #106730)

[Erich Keane via cfe-commits]

================
@@ -4393,17 +4393,47 @@ void SubstTemplateTypeParmType::Profile(llvm::FoldingSetNodeID &ID,
   ID.AddBoolean(Final);
 }
 
+SubstPackType::SubstPackType(TypeClass Derived, QualType Canon,
+                             const TemplateArgument &ArgPack)
+    : Type(Derived, Canon,
+           TypeDependence::DependentInstantiation |
+               TypeDependence::UnexpandedPack),
+      Arguments(ArgPack.pack_begin()) {
+#ifndef NDEBUG
+  for (const auto &P : ArgPack.pack_elements()) {
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erichkeane wrote:

This looks like an:

```suggestion
assert(llvm::all_of(ArgPack, [](auto &P) { return P.getKind() == TemplateArgument::Type; }));
```

https://github.com/llvm/llvm-project/pull/106730