[clang] [Clang][AST] Fix PackIndexingExpr AST printout (PR #117947)

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cor3ntin wrote:

This change needs a release note.
Please add an entry to `clang/docs/ReleaseNotes.rst` in the section the most adapted to the change, and referencing any Github issue this change fixes.


https://github.com/llvm/llvm-project/pull/117947