[clang] [clang-tools-extra] [Clang] Add __type_list_dedup builtin to deduplicate types in template arguments (PR #106730)

[Ilya Biryukov via cfe-commits]

================
@@ -309,7 +309,10 @@ enum BuiltinTemplateKind : int {
   BTK__make_integer_seq,
 
   /// This names the __type_pack_element BuiltinTemplateDecl.
-  BTK__type_pack_element
+  BTK__type_pack_element,
+
+  /// This names the __type_list_dedup BuiltinTemplateDecl.
+  BTK__type_list_dedup,
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ilya-biryukov wrote:

I have mentioned that in the RFC, see [alternatives considered](https://discourse.llvm.org/t/rfc-adding-builtin-for-deduplicating-type-lists/80986#p-328944-alternative-considered-type-trait-4) section.

I think the builtin template is better because it:
- provides the most obvious semantics for type aliases and avoids confusion,
- avoids the need for instantiation of the underlying template with non-duplicated list types,
- is simpler to implement.

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