[clang] [clang][Sema] fix crash on __type_pack_element with dependent packs (GH180307) (PR #180407)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

Independently of this change, we should probably deprecate `__type_pack_element` entirely. @zyn0217 and myself spent quite making sure pack indexing (`Ts...[42]`) handles dependent types properly and I suspect `__type_pack_element`  falls short in more than one instance

https://github.com/llvm/llvm-project/pull/180407