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>

src/tools/clangbackend/ipcsource/clangrequestdocumentannotationsjob.cpp

@@ -40,7 +40,8 @@ static RequestDocumentAnnotationsJob::AsyncResult runAsyncHelper(
 
     RequestDocumentAnnotationsJob::AsyncResult asyncResult;
 
-    translationUnit.extractDocumentAnnotations(asyncResult.diagnostics,
+    translationUnit.extractDocumentAnnotations(asyncResult.firstHeaderErrorDiagnostic,
+                                               asyncResult.diagnostics,
                                                asyncResult.highlightingMarks,
                                                asyncResult.skippedSourceRanges);
 
@@ -83,6 +84,7 @@ void RequestDocumentAnnotationsJob::sendAnnotations(
 {
     const DocumentAnnotationsChangedMessage message(m_pinnedFileContainer,
                                                     result.diagnostics,
+                                                    result.firstHeaderErrorDiagnostic,
                                                     result.highlightingMarks,
                                                     result.skippedSourceRanges);