Fix up FunctionRefs that do not have a signature

[ewoutkramer]

We are becoming more dependent on the signature being provided in the ELM, however in current ELM out there, the signature is not always there.

We will need to add new logic to our ELM pre-processing step to catch the functionrefs where no signature is provided and invoke the logic in our cql2elm compiler to resolve that functionref and add a signature in retrospect.

To make this repeatable, we will need to identify the resolution logic in the compiler, and create a function like

FunctionDef? ResolveOverload(FunctionRef missingSignature, FunctionDef[] allOverloads)

Then, the preprocessor can invoke this logic.