Transform functions returning an aggregate to functions returning multiple values

[Lancern]

Have you considered making ABI lowering to transform calls to return two/N values instead? LLVM IR instructions does not support multiple results and using structs is the way they go about implementing this, MLIR knows better though.

Originally posted by @bcardosolopes in #1292 (comment)

[bcardosolopes]

I'm not 100% it's the best solution, but it could be :)