"function runtime is unable to start" when trying to add Mvc Core

Hi,
I am trying to use Razor Engine as a templating engine in a V2 function. I have it running locally, but not when deployed as a function. When selecting the function name from the portal, it gives "The function runtime is unable to start."

I've managed to strip everything down to a sample function with bare code and tried a lot of combinations with the basic initialization code, but not resolved.

#### Investigative information

Please provide the following:

- Timestamp: 2019-11-05T11:10:59.502Z
- Function App version (1.0 or 2.0): V2
- Session ID: 3735937f63d6477e826a152120f99a3e
- Region: West Europe
No invocation id, the runtime does not start.

#### Repro steps

I've built a sample HttpTrigger function from VS2019 template, and just added this:

```csharp
[assembly: FunctionsStartup(typeof(TestFunction.Startup))]
namespace TestFunction
{
public class Startup : FunctionsStartup
{
public override void Configure(IFunctionsHostBuilder builder)
{
ConfigureServices(builder.Services);
}

private void ConfigureServices(IServiceCollection services)
{
// This line causes error in runtime
services.AddMvcCore();
}
}
}
```

I also tried with a more complex startup adding logging etc. but it gives the same error on the "AddMvcCore" line. If I comment that line, it works.

#### Actual behavior

Function does not run. Runtime does not start.
Navigating to the function name tab in the portal shows a "function runtime is unable to start" error.

#### Related information

Provide any related information

* Programming language used: C#
* Links to source: Can provide them if required.
* Bindings used: HttpTrigger, not relevant. Same problem with QueueTrigger

Start with the V2 HttpTrigger sample and the FunctionsStartup.ConfigureServices method shown in the report, focusing on the services.AddMvcCore() call that prevents the deployed runtime from starting. Reproduce it locally and when deployed, then confirm that the function runtime starts with AddMvcCore enabled and that the portal no longer reports a startup failure.

Written by the indexing model from the issue text.