Fix rb_define_method(singleton_class_of_module, ...) not to change nested module name

Always track class and singleton class stored to a variable to `@classes`.
Otherwise, `find_class var_name, class_name` called from `handle_method` will add a new class to `@toplevel`.

Author: tompng

lib/rdoc/parser/c.rb

@@ -1054,6 +1054,7 @@ def handle_method(type, var_name, meth_name, function, param_count,
   def handle_singleton(sclass_var, class_var)
     class_name = @known_classes[class_var]
 
+    @classes[sclass_var]           = @classes[class_var]
     @known_classes[sclass_var]     = class_name
     @singleton_classes[sclass_var] = class_name
   end

test/rdoc/parser/c_test.rb

@@ -781,6 +781,29 @@ def test_do_methods_singleton_class
     assert               methods.first.singleton
   end
 
+
+  def test_do_methods_nested_module_singleton_class
+    parser = util_parser <<~EOF
+      VALUE baz(VALUE klass, VALUE year) {
+      }
+      void Init_Foo(void) {
+        VALUE mFoo = rb_define_module("Foo");
+        VALUE mBar = rb_define_module_under(mFoo, "Bar");
+        VALUE mBarS = rb_singleton_class(mBar);
+        rb_define_method(mBarS, "baz", baz, 0);
+      }
+    EOF
+    parser.scan
+
+    klass = parser.classes['mBarS']
+    assert_equal 'Foo::Bar', klass.full_name
+    assert_equal 'Foo::Bar', parser.singleton_classes['mBarS']
+    methods = klass.method_list
+    assert_equal 1,      methods.length
+    assert_equal 'baz',  methods.first.name
+    assert               methods.first.singleton
+  end
+
   def test_do_missing
     parser = util_parser
 
@@ -1973,7 +1996,8 @@ def test_scan
     expected = {
       @fn => {
         'mM' => 'M',
-        'cC' => 'C', }}
+        'cC' => 'C',
+        'sC' => 'C' }}
     assert_equal expected, @store.c_class_variables
 
     expected = {