PluisExpression typecheck method does not assign its type when it's operand types are strings

When the first expression of a plus operation is not a String literal a PlusExpression is created during parsing instead of StringPlusExpression. During type checking if the first operand turns out to have type String rather than a numeric type then the second argument is type checked against String and the result type is determined to be a String. However, the type check method fails to assign the type leaving it as TYPE.Undefined. If this expression is consumed by another expression it causes a type check failure.