Clang: "Remove All Splits" leads to ignored/postponed clangbackend jobs (no reparse, completion, ...)

Reproducible with

1. Open two files/documents side by side
2. Use Menu: Window -> Remove All Splits
   ==> From now on all clangbackend jobs are ignored/postponed (no reparse, no completion, ...)

Observations:

1. ModelManagerSupportClang::ModelManagerSupportClang() connects to Core::EditorManager::editorsClosed
2. In the corresponding slot Core::EditorManager::visibleEditors() is called, which yields an empty list for the "Remove All Splits" case - as a result no visible documents are propagated to the clangbackend and so it ignored all jobs. Expected: Since one document is visible afterwards, it should be reported as visible here, too.

This happens since

commit 80ae992c91985389f6d870ac0201c16d895bc5cc
Clang: Fix another crash when closing document

where the connection was changed from queued to normal.