loplugin:unusedfields fix false positive

When the field in question is read from inside a constructor
initializer.

In the process, create some needed infrastructure in the plugin classes.

Change-Id: I2f440efa6912801a236727c9fe3180404616958c
Reviewed-on: https://gerrit.libreoffice.org/38960
Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

compilerplugins/clang/plugin.hxx

@@ -78,6 +78,8 @@ class Plugin
         bool isInUnoIncludeFile(const FunctionDecl*) const;
 
         static void normalizeDotDotInFilePath(std::string&);
+
+        static bool isUnitTestMode();
     private:
         static void registerPlugin( Plugin* (*create)( const InstantiationData& ), const char* optionName, bool isPPCallback, bool byDefault );
         template< typename T > static Plugin* createHelper( const InstantiationData& data );