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

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

Some nits, but I like where this is going.
I'll be away for the rest of the week, but I'm sure @erichkeane @mizvekov and @zyn0217 can help progress the review :)

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