Not all functions have a return slot, and once we have composite forms, functions will be able to have any number of return slots. Obtaining a unique return slot for a function only makes sense in `returned var` handling.
Destroy
CustomWitness
import Cpp;
Cpp
import Cpp inline "some code";
CppCompat.ULong32
CppCompat.LongLong64
CppCompat.ULongLong64
intN_t
uintN_t
none.carbon
CppVoidType