CppTools: Auto-include pre-compiled headers

So far the pre-compiled headers were processed (thus defines from those
headers were visible), but the actual includes for the documents were
not added, which is necessary for lookup/completion.

Note that this will be only done if pre-compiled headers are not ignored
(Options > C++ > Code Model > [] Ignore pre-compiled headers).

Change-Id: I54a8e6b00597af164d958e3e9f2a1075ea187788
Reviewed-by: Erik Verbruggen <erik.verbruggen@digia.com>

src/plugins/cpptools/cppsnapshotupdater.cpp

@@ -177,7 +177,8 @@ void SnapshotUpdater::update(CppModelManager::WorkingCopy workingCopy)
         }
         if (!m_editorDefines.isEmpty())
             sourceProcessor.run(editorDefinesFileName);
-        sourceProcessor.run(m_fileInEditor);
+        sourceProcessor.run(m_fileInEditor, m_usePrecompiledHeaders ? m_precompiledHeaders
+                                                                    : QStringList());
 
         m_snapshot = sourceProcessor.snapshot();
         Snapshot newSnapshot = m_snapshot.simplified(document());