CPlusPlus: Add new usage tag "Template"

For use in follow-up patch.

Change-Id: I49c057280be9b09862a89fa385a7396f1b1093bb
Reviewed-by: Christian Stenger <christian.stenger@qt.io>

src/libs/cplusplus/FindUsages.h

@@ -24,6 +24,7 @@ public:
         WritableRef = 1 << 3,
         Override = 1 << 4,
         MocInvokable = 1 << 5,
+        Template = 1 << 6,
     };
     using Tags = QFlags<Tag>;