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