We previously doubled the id-expression expansion, even when the pack
was expanded to empty. The previous condition for determining whether
we should expand couldn't distinguish between cases where 'the expansion
was previously postponed' and 'the expansion occurred but resulted in
emptiness.'

In the latter scenario, we crash because we have not been examining the
current lambda's parent local instantiation scope since D98068: Any
Decls instantiated in the parent scope are not visible to the generic
lambda. And thus any attempt of looking for instantiated Decls in the lambda
is capped to the current Lambda's LIS.

Fixes llvm#92230