[clang] [libcxxabi] [llvm] [Clang] Mangling of pack indexing type and expression for itanium (PR #123513)

[Shafik Yaghmour via llvm-commits]

https://github.com/shafik commented:

This makes sense but let's make sure we cover all the added branches in the test suite.

https://github.com/llvm/llvm-project/pull/123513