[ASTGen] Add a convenient method to process attribute arguments

[rintaro]

generateWithLabeledExprListArguments(attribute:_:) ensures all the LabeledExprListSyntax arguments are processed in the generator function.

It simplifies:

    guard var args = node.arguments?.as(LabeledExprListSyntax.self)?[...] else {
      self.diagnose(.expectedArgumentsForAttribtue(node))
      return nil
    }
    
    ...consume arguments...   

    if !args.isEmpty {
      self.diagnose(.extraneousArgumentsInAttribute(node, args.first!))
    }

into:

    self.generateWithLabeledExprListArguments(attribute: node) { args in
      ...consume arguments...
    }

[rintaro]

@swift-ci Please smoke test

[hamishknight]

Nice

[rintaro]

@swift-ci Please smoke test

[rintaro]

@swift-ci Please smoke test