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

[Richard Smith via cfe-commits]

https://github.com/zygoloid commented:

This approach makes sense to me. I'd intended that `__type_pack_element` would be the start of a family of builtin templates operating on packs, but sadly didn't get very far down that road.

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