Set the bit that will override binding redirects

In Update 1, devenv.exe.config specifies the binding redirects for
the Roslyn assemblies, in addition to us specifying them in the pkgdef.
If you're building an experimental VSIX, we need to specify a temporary
flag which will enable the shell to ignore the other redirects in
the .exe.config in favor of our pkgdef ones.

src/VisualStudio/Setup/ProvideRoslynBindingRedirection.cs

@@ -53,11 +53,18 @@ public ProvideRoslynBindingRedirectionAttribute(string assemblyName)
         public override void Register(RegistrationContext context)
         {
             _redirectionAttribute.Register(context);
+
+            // Opt into overriding the devenv.exe.config binding redirect
+            using (var key = context.CreateKey(@"RuntimeConfiguration\dependentAssembly\bindingRedirection\" + _redirectionAttribute.Guid.ToString("B").ToUpperInvariant()))
+            {
+                key.SetValue("isPkgDefOverrideEnabled", true);
+            }
         }
 
         public override void Unregister(RegistrationContext context)
         {
             _redirectionAttribute.Unregister(context);
         }
+
     }
 }