swift234

[sil-serializer] Do not use RPOT order for serializing SIL basic blocks

My earlier patch started serializing SIL basic blocks using the RPOT order. While it works, changing the existing order of BBs during the serialization may be very surprising for users. After all, serialization is not supposed to transform the code.

Therefore, this patch follows a different approach. It uses the existing order of BBs during the serialization. When it deserializes SIL and detects a use of an opened archetype before its definition, it basically introduced a forward definition of this opened archetype. Later on, when the actual definition of the opened archetype is found, it replaces the forward definition. There is a correctness check at the end of a SIL function deserialization, which verifies that there are no forward definitions of opened archetypes left unresoved.