C++: Small fixes to the token buffer.

- Fixed blocking macro check
- Enabled buffer "compression": when tokens are generated and no new
macro is being blocked, then prepend the tokens to the previous buffer.
This happens a lot when undo-ing look-ahead.
- Added documentation

Change-Id: I6fa816d94ce4696e473bdbc4f3bf477d77e4dd51
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>