CppEditor: copy snapshot before FunctionDeclDefLink change calculation.

FunctionDeclDefLink::changes is called from
CPPEditorWidget::updateFunctionDeclDefLinkNow() where it receives the
snapshot by reference from the semantic info. The semantic info is
recalculated in a/the future, so it might change the snapshot or its
documents while it's still being used by the decl/def link finder.

So, this patch takes a copy of all relevant semantic info (snapshot and
document) before starting to calculate the changes.

Change-Id: I78244a4ca8149233403b3c35ee05a2d4ed4b2770
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@digia.com>

src/plugins/cppeditor/cppeditor.cpp

@@ -1029,22 +1029,24 @@ void CPPEditorWidget::updateFunctionDeclDefLinkNow()
 {
     if (Core::EditorManager::currentEditor() != editor())
         return;
+    const Snapshot semanticSnapshot = d->m_lastSemanticInfo.snapshot;
+    const Document::Ptr semanticDoc = d->m_lastSemanticInfo.doc;
     if (d->m_declDefLink) {
         // update the change marker
-        const Utils::ChangeSet changes = d->m_declDefLink->changes(d->m_lastSemanticInfo.snapshot);
+        const Utils::ChangeSet changes = d->m_declDefLink->changes(semanticSnapshot);
         if (changes.isEmpty())
             d->m_declDefLink->hideMarker(this);
         else
             d->m_declDefLink->showMarker(this);
         return;
     }
-    if (!d->m_lastSemanticInfo.doc || isOutdated())
+    if (semanticDoc.isNull() || isOutdated())
         return;
 
     Snapshot snapshot = CppModelManagerInterface::instance()->snapshot();
-    snapshot.insert(d->m_lastSemanticInfo.doc);
+    snapshot.insert(semanticDoc);
 
-    d->m_declDefLinkFinder->startFindLinkAt(textCursor(), d->m_lastSemanticInfo.doc, snapshot);
+    d->m_declDefLinkFinder->startFindLinkAt(textCursor(), semanticDoc, snapshot);
 }
 
 void CPPEditorWidget::onFunctionDeclDefLinkFound(QSharedPointer<FunctionDeclDefLink> link)