Update return type in code generator and add tests for logical functions

Change-Id: I128f8ccc21c2c3a2c79674ac28dd9a5b26b58772

include/ceres/codegen/internal/code_generator.h

@@ -102,9 +102,6 @@ class CodeGenerator {
   // If the expression does not have a valid name an error is generated.
   std::string VariableForExpressionId(ExpressionId id);
 
-  // Returns the type as a string of the left hand side.
-  static std::string DataTypeForExpression(ExpressionType type);
-
   // Adds one level of indentation. Called when an IF expression is encountered.
   void PushIndentation();