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.cxx

@@ -265,6 +265,11 @@ SourceLocation Plugin::locationAfterToken( SourceLocation location )
     return Lexer::getLocForEndOfToken( location, 0, compiler.getSourceManager(), compiler.getLangOpts());
     }
 
+bool Plugin::isUnitTestMode()
+    {
+    return PluginHandler::isUnitTestMode();
+    }
+
 RewritePlugin::RewritePlugin( const InstantiationData& data )
     : Plugin( data )
     , rewriter( data.rewriter )