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

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin approved this pull request.

LGTM
Please give it another ~24h  in case someone else has anything to add.

Thanks for the huge amount of work done here, this will be super useful.

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