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/pluginhandler.hxx

@@ -37,6 +37,7 @@ class PluginHandler
         DiagnosticBuilder report( DiagnosticsEngine::Level level, const char * plugin, StringRef message,
             CompilerInstance& compiler, SourceLocation loc = SourceLocation());
         bool addRemoval( SourceLocation loc );
+        static bool isUnitTestMode();
     private:
         void handleOption( const string& option );
         void createPlugins( set< string > rewriters );
@@ -47,7 +48,6 @@ class PluginHandler
         string scope;
         string warningsOnly;
         bool warningsAsErrors;
-        bool unitTestMode;
     };
 
 /**