swift234

IRGen: Add ResilienceExpansion parameter to TypeInfo::isKnownEmpty(), NFC

In a few places, we have to be careful about the distinction between
"empty in this resilience domain" versus "empty in all resilience
domains". Make callers think about this by adding a parameter instead
of relying on them to check isFixedSize() as necessary first.

While making this change I noticed that the code for checking if
types are empty when computing extra inhabitants of structs and enums
might be slightly wrong in the face of resilience; I will revisit
this later.