Bug 1153348 - Add an analysis to prohibit operator bools which aren't marked as either explicit or MOZ_IMPLICIT; r=jrmuizel

This is the counterpart to the existing analysis to catch
constructors which aren't marked as either explicit or
MOZ_IMPLICIT.

dom/plugins/base/nsNPAPIPluginInstance.cpp

@@ -1167,7 +1167,7 @@ public:
     if (plugin)
       mLibrary = plugin->GetLibrary();
   }
-  operator bool() { return !!mLibrary; }
+  explicit operator bool() { return !!mLibrary; }
   PluginLibrary* operator->() { return mLibrary; }
 
 private: