[Sema] Avoid an undesired pack expansion while transforming PackIndexingType (#90195)

A pack indexing type can appear in a larger pack expansion, e.g
`Pack...[pack_of_indexes]...` so we need to temporarily disable
substitution of pack elements.

Besides, this patch also fixes an assertion failure in
`PackIndexingExpr::classify`: dependent `PackIndexingExpr`s are always
LValues and thus we don't need to consider their `IndexExpr`s.

Fixes https://github.com/llvm/llvm-project/issues/88925



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Co-authored-by: cor3ntin <corentinjabot@gmail.com>