Scaffolding fails with partial methods and lang version 9.0

### Describe the bug
We have a project that uses a source code generator. We are trying to scaffold Identity pages with VS UI. Scaffolding interface give following error:
```
Error

There was an error running the selected code generator:
'Failed to compile the project in memory
C:\Users\user\project\project.Web\Managers\AccountManager.cs(64,44): error CS0752: A partial method cannot have out parameters
C:\Users\user\project\project.Web\Managers\AccountManager.cs(64,44): error CS0766: Partial methods must have a void return type
```
Lang Version 9.0

### Further technical details
- ASP.NET Core version 3.1.14
- The IDE VS 2019 16.9.3

Reproduce Identity page scaffolding in the reported ASP.NET Core 3.1.14 and Visual Studio 2019 16.9.3 setup with language version 9.0. Start by tracing the in-memory compilation that reports the partial-method errors in Managers/AccountManager.cs. Done means scaffolding completes without those compiler errors.

Written by the indexing model from the issue text.