C++: add fallback project part querying

When a file is not part of any other project part, this project part can
be used to get at least some compiler flags, defines, etc. to use. This
can happen when either a file outside a project is opened, or when the
project description is incomplete.

Change-Id: I5d595fae7195e8b61dbad14368b6cae9eb15c21b
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>