Build Design project's parent when not under solution build

The reason we build the Controls library first is that we have a dependency on the XML Doc file it generates.
The Doc file is embedded within the Design library and it uses it's type-info to populate metadata automatically.

build/Windows.Toolkit.VisualStudio.Design.targets

@@ -5,6 +5,14 @@
     <OutputPath>$(ParentProjectOutputPath)Design\</OutputPath>
   </PropertyGroup>
 
+  <ItemGroup Condition="'$(SolutionDir)' == '' OR '$(SolutionDir)' == '*Undefined*'">
+    <ProjectReference Include="..\$(ParentProjectName)\$(ParentProjectName).csproj">
+      <Private>False</Private>
+      <ReferenceOutputAssembly>False</ReferenceOutputAssembly>
+      <SkipGetTargetFrameworkProperties>True</SkipGetTargetFrameworkProperties>
+    </ProjectReference>
+  </ItemGroup>
+
   <ItemGroup>
     <PackageReference Include="Microsoft.VisualStudio.DesignTools.Extensibility" Version="16.4.29519.181" ExcludeAssets="Runtime" />
     <PackageReference Include="Microsoft.Windows.SDK.Contracts" Version="$(TargetPlatformBaseVersion).$(TargetPlatformMinRevision).*" />