Fix build XamlBuildTask failure on 64-bit MSBuild (dotnet#2001) (dotn…

…et#2020) (dotnet#2059)

Allows 64-bit MSBuild to find XamlBuildTask.dll to build 64-bit Xaml projects.

Related to internal issue DevDiv.372915.

src/MSBuild/app.config

@@ -41,10 +41,12 @@
         <dependentAssembly>
           <assemblyIdentity name="Microsoft.Activities.Build" culture="neutral" publicKeyToken="31bf3856ad364e35" />
           <bindingRedirect oldVersion="4.0.0.0" newVersion="15.0.0.0" />
+          <codeBase version="15.0.0.0" href=".\amd64\Microsoft.Activities.Build.dll" />
         </dependentAssembly>
         <dependentAssembly>
           <assemblyIdentity name="XamlBuildTask" culture="neutral" publicKeyToken="31bf3856ad364e35" />
           <bindingRedirect oldVersion="4.0.0.0" newVersion="15.0.0.0" />
+          <codeBase version="15.0.0.0" href=".\amd64\XamlBuildTask.dll" />
         </dependentAssembly>
 
         <!-- Workaround for crash in C++ CodeAnalysis scenarios due to https://github.com/Microsoft/msbuild/issues/1675 -->

src/Tasks/Microsoft.Xaml.targets

@@ -13,7 +13,7 @@
         the .NET Framework directory and thus will no longer be the right answer. Override it to point
         to the correct .NET Framework location. --> 
    <PropertyGroup>
-      <XamlBuildTaskPath Condition="'$(XamlBuildTaskPath)' == ''">$(MSBuildThisFileDirectory)</XamlBuildTaskPath>
+      <XamlBuildTaskPath Condition="'$(XamlBuildTaskPath)' == ''">$(MSBuildToolsPath64)</XamlBuildTaskPath>
    </PropertyGroup>
 
    <Import Project="$(MSBuildFrameworkToolsPath)\Microsoft.Xaml.targets" Condition="Exists('$(MSBuildFrameworkToolsPath)\Microsoft.Xaml.targets')" />