Runtime support for upcoming .NET Core WinRT Host (dotnet/coreclr#23402)
* First pass at adding winrt host entry-point.
* There's no way to specify HResult-swapping on a function called via hosted-interop so just return the HResult instead.
* Use the WindowsRuntimeMarshal class to create the activation factory and initialize it.
* Implement loading the dependent assemblies of a WinRT assembly into an isolated load context.
* PR Feedback.
* Fail to get the activation factory if the found type is not a managed type exported from a winmd.
* Rearrange parameters based on PR feedback.
* Remove unneeded include.
* Make ActivationFactoryLoader internal.
* Fix null-ref in WinRT-dependent-assembly loading
* Remove extraneous "System."
Commit migrated from https://github.com/dotnet/coreclr/commit/
fdc9998609263d9f531079e2cbe0f3f31eba3382