Make every operator bool() explicit

operator bool() is a trap, but with explicit it's far safer,
and we can use that now.

Change-Id: I4e58631c94e87c00256c3ab3cff4fd2c5f632713
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@theqtcompany.com>

src/plugins/coreplugin/vcsmanager.cpp

@@ -511,7 +511,7 @@ public:
         ExtensionSystem::PluginManager::addObject(watch);
     }
 
-    operator bool() { return m_watched; }
+    explicit operator bool() { return m_watched; }
     bool operator !() { return !m_watched; }
     T &operator*() { return *m_watched; }
     T *operator->() { return m_watched; }