Ensure dotnet-ilrepack is installed before IL merging

This simplifies consumption and avoids having to create custom CI build steps to install the right tool.

If the PR to include this global tool in the main ILRepack distribution is merged (see gluck/il-repack#364), we can remove the --add-source and --prerelease switches.

samples/dotnet/SponsorLink.targets

@@ -115,6 +115,20 @@ partial class SponsorLink
     </ItemGroup>
   </Target>
 
+  <Target Name="EnsureILRepack" BeforeTargets="ILRepack" Condition="'$(MergeAnalyzerAssemblies)' == 'true'">
+    <Exec Command="ilrepack --version" StandardErrorImportance="high" StandardOutputImportance="low" ConsoleToMSBuild="true" IgnoreExitCode="true" ContinueOnError="true">
+      <Output TaskParameter="ConsoleOutput" PropertyName="ILRepackOutput" />
+      <Output TaskParameter="ExitCode" PropertyName="ExitCode" />
+    </Exec>
+    <Message Importance="high" Text="Using installed dotnet-ilrepack v$(ILRepackOutput)" Condition="$(ExitCode) == '0'" />
+    <Exec Command="dotnet tool install -g dotnet-ilrepack --prerelease --add-source https://kzu.blob.core.windows.net/nuget/index.json"
+          Condition="$(ExitCode) != '0'" />
+    <Exec Command="ilrepack --version" Condition="$(ExitCode) != '0'" ConsoleToMSBuild="true">
+      <Output TaskParameter="ConsoleOutput" PropertyName="ILRepackInstalledOutput" />
+    </Exec>
+    <Message Importance="high" Text="Installed dotnet-ilrepack v$(ILRepackInstalledOutput)" Condition="$(ExitCode) != '0'" />
+  </Target>
+
   <Target Name="ILRepack" AfterTargets="CoreCompile" BeforeTargets="CopyFilesToOutputDirectory"
           Inputs="@(IntermediateAssembly -&gt; '%(FullPath)')"
           Outputs="$(IntermediateOutputPath)ilrepack.txt"