slicc: enable overloading in functions not in classes

slicc: enable overloading in functions not in classes

For many years the slicc symbol table has supported overloaded functions in
external classes.  This patch extends that support to functions that are not
part of classes (a.k.a. no parent).  For example, this support allows slicc
to understand that mapAddressToRange is overloaded and the NodeID is an
optional parameter.