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

[Ilya Biryukov via cfe-commits]

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@@ -1510,6 +1517,21 @@ namespace {
       }
     }
 
+    MultiLevelTemplateArgumentList ForgetSubstitution() {
+      MultiLevelTemplateArgumentList New;
+      New.addOuterRetainedLevels(this->TemplateArgs.getNumLevels());
+
+      MultiLevelTemplateArgumentList Old =
+          const_cast<MultiLevelTemplateArgumentList &>(this->TemplateArgs);
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ilya-biryukov wrote:

Unfortunately yes, but it's the same as in `ForgetPartiallySubstitutedPack/RememberPartiallySubstitutedPack`.
`this->TemplateArgs` is a const reference, and it's probably even good to have some protection against changing it accidentally in **most** places, except these two functions.

We might want to change this, but it's not a pattern that this change adds and so I'd keep this separate from the PR.

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