Clang: Add placeholder text to Clang Query

Task-number: QTCREATORBUG-18815
Change-Id: Ie5d53f0b3f2464408bb09022483a3a6258e8173d
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

src/plugins/clangrefactoring/clangquerytexteditorwidget.cpp

@@ -38,6 +38,7 @@ ClangQueryTextEditorWidget::ClangQueryTextEditorWidget(QWidget *parent)
       m_hoverHandler(std::make_unique<ClangQueryHoverHandler>(m_syntaxHighlighter))
 {
     textDocument()->setSyntaxHighlighter(m_syntaxHighlighter);
+    textDocument()->setPlainText("functionDecl()");
 
     addHoverHandler(m_hoverHandler.get());
 }