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

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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?

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