Misc idl_gen_cpp cleanup

- Update to be const-correct where possible.
- Consistently pass |code| as pointer instead of non-const-ref.
- No newlines (\n) characters in the middle of code strings.
- Use if-else if-else statements instead of nested ternary operators.
- Ensure all lines end at 80 chars.
- Make utility functions static.

From cl/143505731.

Change-Id: If0fab9ee75de5af963367a948dddf53af93f73b4

src/idl_gen_general.cpp

@@ -276,7 +276,7 @@ class GeneralGenerator : public BaseGenerator {
     return SaveFile(filename.c_str(), code, false);
   }
 
-  const Namespace *CurrentNameSpace() { return cur_name_space_; }
+  const Namespace *CurrentNameSpace() const { return cur_name_space_; }
 
   std::string FunctionStart(char upper) {
     return std::string() + (lang_.language == IDLOptions::kJava