Fix an error for Rails/Delegate

This PR fixes the following error for `Rails/Delegate` when delegation to a cbase namespaced constant.

```console
  1) RuboCop::Cop::Rails::Delegate detects delegation to a cbase namespaced constant
     Failure/Error: return node.source unless node.namespace

     NoMethodError:
       undefined method 'namespace' for an instance of RuboCop::AST::Node
     # ./lib/rubocop/cop/rails/delegate.rb:113:in 'RuboCop::Cop::Rails::Delegate#full_const_name'
     # ./lib/rubocop/cop/rails/delegate.rb:115:in 'RuboCop::Cop::Rails::Delegate#full_const_name'
     # ./lib/rubocop/cop/rails/delegate.rb:115:in 'RuboCop::Cop::Rails::Delegate#full_const_name'
```

Follow-up to #1438.

There is no changelog entry since this is a bug fix for the above PR that has not been released yet.

lib/rubocop/cop/rails/delegate.rb

@@ -110,9 +110,12 @@ def build_delegation(node, receiver)
         end
 
         def full_const_name(node)
-          return node.source unless node.namespace
+          return unless node.const_type?
+          unless node.namespace
+            return node.absolute? ? "::#{node.source}" : node.source
+          end
 
-          "#{full_const_name(node.namespace)}::#{node.children.last}"
+          "#{full_const_name(node.namespace)}::#{node.short_name}"
         end
 
         def trivial_delegate?(def_node)

spec/rubocop/cop/rails/delegate_spec.rb

@@ -280,6 +280,19 @@ def foo
     RUBY
   end
 
+  it 'detects delegation to a cbase namespaced constant' do
+    expect_offense(<<~RUBY)
+      def foo
+      ^^^ Use `delegate` to define delegations.
+        ::SomeModule::CONST.foo
+      end
+    RUBY
+
+    expect_correction(<<~RUBY)
+      delegate :foo, to: :'::SomeModule::CONST'
+    RUBY
+  end
+
   it 'detects delegation to an instance variable' do
     expect_offense(<<~RUBY)
       def foo