C++: Only parse with appropriate defines for open editors.

If two files from different (sub-)projects include the same header file,
and the defined macros differ for both files, the header file will be
parsed with only the appropriate macros for the including file.

Task-number: QTCREATORBUG-9802
Task-number: QTCREATORBUG-1249

Change-Id: I560490afa287b3bb1e863bce1bb4f57af36ad56e
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>