Do not warn on captured underscored vars from defmodule

Closes #12262

Author: josevalim

lib/elixir/lib/kernel.ex

@@ -4806,7 +4806,7 @@ defmodule Kernel do
     var_meta =
       case prune do
         true -> [generated: true, keep_unused: true]
-        false -> [generated: false]
+        false -> [generated: true]
       end
 
     module_vars = :lists.map(&module_var(&1, var_meta), :maps.keys(versioned_vars))

lib/elixir/test/elixir/module_test.exs

@@ -332,6 +332,13 @@ defmodule ModuleTest do
     assert Elixir.ModuleTest.NonAtomAlias.hello() == :world
   end
 
+  test "does not warn on captured underscored vars" do
+    _unused = 123
+
+    defmodule __MODULE__.NoVarWarning do
+    end
+  end
+
   @compile {:no_warn_undefined, ModuleCreateSample}
 
   test "create" do