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

tests/namespace_test/namespace_test1_generated.h

@@ -61,7 +61,9 @@ struct TableInNestedNS FLATBUFFERS_FINAL_CLASS : private flatbuffers::Table {
 struct TableInNestedNSBuilder {
   flatbuffers::FlatBufferBuilder &fbb_;
   flatbuffers::uoffset_t start_;
-  void add_foo(int32_t foo) { fbb_.AddElement<int32_t>(TableInNestedNS::VT_FOO, foo, 0); }
+  void add_foo(int32_t foo) {
+    fbb_.AddElement<int32_t>(TableInNestedNS::VT_FOO, foo, 0);
+  }
   TableInNestedNSBuilder(flatbuffers::FlatBufferBuilder &_fbb) : fbb_(_fbb) { start_ = fbb_.StartTable(); }
   TableInNestedNSBuilder &operator=(const TableInNestedNSBuilder &);
   flatbuffers::Offset<TableInNestedNS> Finish() {