Allow non-concrete classes when marshalling SafeHandle by-value

[elinor-fung]

The source generator currently requires a non-abstract subclass of SafeHandle - even for by-value parameters. We require a concrete subclass to instantiate when handling out / ref, but shouldn't need it for by-value.

Declaring a P/Invoke using an abstract base class or SafeHandle itself is a common use case in runtime libraries - e.g:
Interop.Read.cs

[DllImport(Libraries.SystemNative, EntryPoint = "SystemNative_Read", SetLastError = true)]
internal static extern unsafe int Read(SafeHandle fd, byte* buffer, int count);

Interop.Properties.cs

[DllImport(Interop.Libraries.NCrypt, CharSet = CharSet.Unicode)]
internal static extern unsafe ErrorCode NCryptGetProperty(SafeNCryptHandle hObject, string pszProperty, [Out] void* pbOutput, int cbOutput, out int pcbResult, CngPropertyOptions dwFlags);

cc @AaronRobinsonMSFT @jkoritzinsky

[jkoritzinsky]

Fixed by #1066