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

[Oliver Hunt via cfe-commits]

https://github.com/ojhunt requested changes to this pull request.

Needs a few more tests, and I think a bunch of the moved dependence checks should be reverted

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