[clang] [clang][serialization] Fix crash on imported pack indexing type (PR #205965)

[via cfe-commits]

IamYJLee wrote:

> I’m not sure why clang/ReleaseNotes.rst was removed. During the rebase, it appeared as a newly added file when I restored it. Could you help me understand why this happened?

Oh..
I found `https://github.com/llvm/llvm-project/pull/206182`

https://github.com/llvm/llvm-project/pull/205965