CppTools: Fix snapshot in case another project is opened

When a second project was opened the snapshot was reset. That resulted
in all kinds of problems since from then on the code model effectively
was not aware of the files of the first project.

The regression was introduced by commit a0d6df7b.

Change-Id: I1ccc9de68177205b49a4ba8ead2bc8abe4592b32
Reviewed-by: Fawzi Mohamed <fawzi.mohamed@digia.com>