Clang: Fix recovering on clangbackend crash

The translation unit was updated, but not re-registered.

Handle the editor documents this way:
  1. Reset all ClangEditorDocumentProcessors (this will send an
     unregister message, but that's not problematic).
  2. For the visible editor documents, run their processors so that the
     translation units will be re-registered.
  3. For the invisible editor documents, mark them dirty. Once the user
     makes an invisible document visible again, the processor will run
     and also re-register the translation unit.

Change-Id: I23693ac197bd34a183f3a0020eb5372268636599
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>