CppTools: Remove separate indexing revision

For indexing we used a custom revision that was updated on each
modelManager BuiltinIndexingSupport::refreshSourceFiles() call. This
could lead to rejection of updated documents triggered by refactoring
actions, like for the following case:

 1. Open a project containing a.h and a.cpp
 2. Open a.cpp, insert some new lines, save and close the document
 3. Open a.h and rename a function that is defined in a.cpp
    --> The refactoring action modifies a.h and a.cpp, so re-indexing
        of those is triggered. Since a.cpp has already a higher revision
        (step 2) than the updated document, the updated document is
        discarded. As a consequence find usages and follow symbol fails
        for the renamed function.

Now the document call back provided to CppSourceProcessor is responsible
for updating the document revision based on the latest revision in the
global snapshot.

Change-Id: I4dfa0a4d34991655acfa749109f00c47b0fbfdbe
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Erik Verbruggen <erik.verbruggen@theqtcompany.com>
Reviewed-by: Eike Ziller <eike.ziller@theqtcompany.com>