[clang][Sema] Avoid guessing unexpanded packs' size in getFullyPackExpandedSize (#87768)

There has been an optimization for `SizeOfPackExprs` since c5452ed9, in
which
we overlooked a case where the template arguments were not yet
formed into a `PackExpansionType` at the token annotation stage. This
led to a problem in that a template involving such expressions may
lose its nature of being dependent, causing some false-positive
diagnostics.

Fixes https://github.com/llvm/llvm-project/issues/84220