Throw errors for nested targets in GN.

tools/gn/functions_target.cc

Index: tools/gn/functions_target.cc
diff --git a/tools/gn/functions_target.cc b/tools/gn/functions_target.cc
index 655c32a5cbbe78466be2264069a73bfc624adf0c..45e685965371f77579b2e25c4a4e0ebd817f3eec 100644
--- a/tools/gn/functions_target.cc
+++ b/tools/gn/functions_target.cc
@@ -31,6 +31,10 @@ Value ExecuteGenericTarget(const char* target_type,
                            const std::vector<Value>& args,
                            BlockNode* block,
                            Err* err) {
+  NonNestableBlock non_nestable(scope, function, "target");
+  if (!non_nestable.Enter(err))
+    return Value();
+
   if (!EnsureNotProcessingImport(function, scope, err) ||
       !EnsureNotProcessingBuildConfig(function, scope, err))
     return Value();