Fix the following use case:
 * Open foo.h
 * Open foo.cpp
 * Change both files externally (e.g. git checkout) e.g. by adding a
   function (declaration + definition)
 * Confirm the reload dialog in Qt Creator
 * The added function in foo.cpp gets a diagnostic that it's out of line
   although foo.h is also reloaded.

TextDocument::contentsChangedWithPosition() is also emitted during a
reload. However, the revision of the document at that point is not yet
updated. This led to an out-dated copy of the unsaved files in the clang
backend.

Change-Id: I70580d32bb7a34dc43356dc05019cc18addf950e
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>