Make [In, Optional] managed struct parameters actually optional in friendly overloads

[AArnott]

Fixes #578

[AArnott]

@AaronRobinsonMSFT .NET Framework interop can handle Unsafe.AsRef<MINIDUMP_CALLBACK_INFORMATION>(null) as an argument to an in MINIDUMP_CALLBACK_INFORMATION parameter of an extern method in order to pass in a null pointer, right? It compiles. I just want to verify that it runs.

[AaronRobinsonMSFT]

.NET Framework interop can handle Unsafe.AsRef<MINIDUMP_CALLBACK_INFORMATION>(null) as an argument to an in MINIDUMP_CALLBACK_INFORMATION parameter of an extern method in order to pass in a null pointer, right?

@AArnott Yes. This will translate into a nullptr/NULL on the native side.

For .NET 5+ we recommend using Unsafe.NullRef<T>, but I assume you don't have that on a .NET Standard 2.1 platform.

[AArnott]

Thanks. The source generator already caters to various TFMs and C# LangVersions, so we can certainly emit Unsafe.NullRef<T> wherever that API is defined.