Change Boogie AST to support unnamed parameters in domain functions.

[fabiopakk]

This task is meant to provide the necessary changes in Carbon in order to implement unnamed parameters in domain functions viperproject/silver#474. Since the original task requires changes in the parse AST and AST, some of the nodes in the Boogie AST need to be changed accordingly as well as its translator. Please refer to the former task for more details.

[fabiopakk]

I've fixed this issue.