Clang: Add API to convert diagnostics to plain text

This is for a follow-up change that will allow to copy the diagnostic
text from the tooltip to the clipboard.

Change-Id: Iad5343a819c84ca83d562f69dcf2f50e1d4785c4
Reviewed-by: Ivan Donchevskii <ivan.donchevskii@qt.io>

src/plugins/clangcodemodel/clangtextmark.cpp

@@ -101,7 +101,8 @@ bool ClangTextMark::addToolTipContent(QLayout *target) const
 {
     using Internal::ClangDiagnosticWidget;
 
-    QWidget *widget = ClangDiagnosticWidget::create({m_diagnostic}, ClangDiagnosticWidget::ToolTip);
+    QWidget *widget = ClangDiagnosticWidget::createWidget({m_diagnostic},
+                                                          ClangDiagnosticWidget::ToolTip);
     target->addWidget(widget);
 
     return true;