ClangCodeModel: Show value in tool tips, if possible

When hovering over a constant expression, it's probably helpful
to show that value to the user.
Requires clang 11 to fully work. For now, it only shows the value for
variable initializations.

Fixes: QTCREATORBUG-23967
Change-Id: I6b844231bac50993c2fa2fa82c552ad9cef590df
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@qt.io>

src/plugins/cpptools/baseeditordocumentprocessor.h

@@ -41,6 +41,8 @@
 
 #include <QTextEdit>
 
+#include <QVariant>
+
 #include <functional>
 
 namespace TextEditor {
@@ -57,6 +59,7 @@ struct CPPTOOLS_EXPORT ToolTipInfo {
     QStringList qDocIdCandidates;
     QString qDocMark;
     Core::HelpItem::Category qDocCategory;
+    QVariant value;
 
     QString sizeInBytes;
 };