Fix MVC :: React to IServiceManfest changes

Take 2: Add BuildFallbackServiceProvider helper to TestHelper which is called whenever a new ServiceProvider is created currently.

@davidfowl Review: Precompilation has its own copy of the code for now.  The building of the fallback service provider code(import, manifest, delegating service provider) is pretty nasty to get right, I still think we should put the common functionality somewhere reusable, since we already have to use the functionality in TestHost, MVC Tests, and MVC itself.  Almost certainly other scenarios are going to crop up in the future where we need this exact same fallback behavior
As we discussed, keeping the workaround code for fallback around for MVC Functional tests, filed https://github.com/aspnet/Mvc/issues/1612 to track removing this