Don't attempt to actually do double code removals

...that easily works around the problem that in a rewriter rewriting types of
VarDecls like

   T x, y;

it would try to replace T twice.  Also, keep the list of removals globally with
the (global) rewriter.

Change-Id: I55b8d11986c2a29e09ff40132fd114a0cc48dc90

compilerplugins/clang/pluginhandler.cxx

@@ -159,6 +159,11 @@ DiagnosticBuilder PluginHandler::report( DiagnosticsEngine::Level level, StringR
     return report( level, nullptr, message, compiler, loc );
     }
 
+bool PluginHandler::addRemoval( SourceLocation loc )
+    {
+    return removals.insert( loc ).second;
+    }
+
 void PluginHandler::HandleTranslationUnit( ASTContext& context )
     {
     if( context.getDiagnostics().hasErrorOccurred())