VariableManager: Do not try to expand Prefixes

Resolving "Prefix:<value>" does not make any sense, so let's not
do that.

Task-number: QTCREATORBUG-15072
Change-Id: Ia9741f9b48d269a7be31163ad73c3bc232856419
Reviewed-by: hjk <hjk@theqtcompany.com>

src/libs/utils/macroexpander.h

@@ -89,6 +89,7 @@ public:
 
     QList<QByteArray> visibleVariables() const;
     QString variableDescription(const QByteArray &variable) const;
+    bool isPrefixVariable(const QByteArray &variable) const;
 
     MacroExpanderProviders subProviders() const;