Add a reference to Microsoft.VisualStudio.Shell.Immutable.$(VisualStudioVersion)

In future versions of Shell.14.0, some types have been type forwarded
to this assembly. This means we'll take a reference to Immutable.14.0
when building with 14.0, but that's fine since we won't use it.

src/VisualStudio/Core/SolutionExplorerShim/SolutionExplorerShim.csproj

@@ -86,6 +86,7 @@
     <Reference Include="Microsoft.VisualStudio.Shell.Immutable.10.0, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"><Private>false</Private></Reference>
     <Reference Include="Microsoft.VisualStudio.Shell.Immutable.11.0, Version=11.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"><Private>false</Private></Reference>
     <Reference Include="Microsoft.VisualStudio.Shell.Immutable.12.0, Version=12.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"><Private>false</Private></Reference>
+    <Reference Include="Microsoft.VisualStudio.Shell.Immutable.$(VisualStudioVersion), Version=$(VisualStudioReferenceAssemblyVersion), Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"><Private>false</Private></Reference>
     <Reference Include="Microsoft.VisualStudio.Shell.Interop, Version=7.1.40304.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a"><Private>false</Private></Reference>
     <Reference Include="Microsoft.VisualStudio.Shell.Interop.10.0, Version=10.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a">
       <EmbedInteropTypes>True</EmbedInteropTypes>