[Clang] Remove the PackExpansion restrictions for rewrite substitution (#126206)
When substituting for rewrite purposes, as in rebuilding constraints for
a synthesized deduction guide, it assumed that packs were in
PackExpansion* form, such that the instantiator could extract a pattern.
For type aliases CTAD, while rebuilding their associated constraints,
this might not be the case because we'll call
`TransformTemplateArgument()` for the alias template arguments, where
there might be cases e.g. a non-pack expansion type into a pack
expansion, so the assumption wouldn't hold.
This patch fixes that by making it treat the non-pack expansions as
direct patterns when rewriting.
Fixes #124715
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