Fix code select for method call with named and positional arguments

[eric-milles]

Consider the following:

def meth(Map args, Class type) {}

meth(name:null, groovy.transform.ToString)
meth(groovy.transform.ToString, name:null)

Code hover (F2) or code select (F3 or Ctrl+click) on "ToString" fails for both methods due to:

CodeSelectRequestor: Could not proceed due to null declaring type for org.codehaus.groovy.ast.expr.MapExpression@115da3[org.codehaus.groovy.ast.expr.MapEntryExpression@11c2fe(key: ConstantExpression[name], value: ConstantExpression[null])]