Recognize GC_PLUGIN_IGNORE() on 'operator new' declarations.

For the select few times an overriding "operator new" is needed for a
GC class, provide the GC_PLUGIN_IGNORE() escape hatch for these. The
responsibility that the "new"'s allocation will be on the Oilpan heap,
rests with the implementor of the class.

R=haraken
BUG=443854

Review URL: https://codereview.chromium.org/975393002

Cr-Commit-Position: refs/heads/master@{#319141}

tools/clang/blink_gc_plugin/BlinkGCPlugin.cpp

@@ -1131,7 +1131,8 @@ class BlinkGCPluginConsumer : public ASTConsumer {
         CheckLeftMostDerived(info);
         CheckDispatch(info);
         if (CXXMethodDecl* newop = info->DeclaresNewOperator())
-          ReportClassOverridesNew(info, newop);
+          if (!Config::IsIgnoreAnnotated(newop))
+            ReportClassOverridesNew(info, newop);
         if (info->IsGCMixinInstance()) {
           // Require that declared GCMixin implementations
           // also provide a trace() override.