Clang: Show info bar for parse errors in header files

...because those errors can lead to a substantial performance/functional
regression.

The actual diagnostics (possibly with children) are shown as details in
the info bar.

The info bar can be hidden with the "Do Not Show Again" button.
Re-enabling the info bar is possible with the new editor tool bar
button.

Change-Id: I03394ff8e3c84127946b0b791930b28a385f5a46
Reviewed-by: David Schulz <david.schulz@qt.io>

tests/unit/unittest/clientserverinprocess-test.cpp

@@ -277,6 +277,7 @@ TEST_F(ClientServerInProcess, SendDocumentAnnotationsChangedMessage)
 
     ClangBackEnd::DocumentAnnotationsChangedMessage message(fileContainer,
                                                             {diagnostic},
+                                                            {},
                                                             {highlightingMark},
                                                             QVector<SourceRangeContainer>());