Throw errors for nested targets in GN.

tools/gn/function_toolchain.cc

Index: tools/gn/function_toolchain.cc
diff --git a/tools/gn/function_toolchain.cc b/tools/gn/function_toolchain.cc
index 2fe5f53a23c5135918262ed2b0a7cea3f9fa7378..4e1fe2d418e684c07616a352b3689b66e2642c7b 100644
--- a/tools/gn/function_toolchain.cc
+++ b/tools/gn/function_toolchain.cc
@@ -301,6 +301,10 @@ Value RunToolchain(Scope* scope,
                    const std::vector<Value>& args,
                    BlockNode* block,
                    Err* err) {
+  NonNestableBlock non_nestable(scope, function, "toolchain");
+  if (!non_nestable.Enter(err))
+    return Value();
+
   if (!EnsureNotProcessingImport(function, scope, err) ||
       !EnsureNotProcessingBuildConfig(function, scope, err))
     return Value();