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

[Ilya Biryukov via cfe-commits]

ilya-biryukov wrote:

> Can you add a release note and some documentation in LanguageExtensions.rst, in the [Type Trait Primitives](https://clang.llvm.org/docs/LanguageExtensions.html#type-trait-primitives) section?

I have added both, PTAL. I opted for the "builtin type aliases" section as this is where other template builtins like `__make_integer_seq` are, so it felt more appropriate

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