[Optional, In] parameter shouldn't use in modifier in extern method for managed structs

[AArnott]

The MiniDumpWriteDump extern method is emitted with an in modifier for its last parameter. While convenient when specifying the struct value, it makes it syntactically impossible to pass in a null pointer to omit the value.
The metadata describes this last parameter as [Optional, In], so there should be a way to pass in a null pointer.

[AArnott]

Very peculiarly, the prior two parameters have the same modifiers, and correctly use pointers in the emitted projection.

[AArnott]

This is because the last parameter's type is managed, since it contains a field that is a delegate type. We could force such parameter types to be unmanaged, or we could force the parameter's type to be an array (of 1) of this struct to allow a null value to be supplied. But the cure may be worse than the disease in either case.

Instead, this should work as a "null" argument, albeit it's undiscoverable: Unsafe.AsRef<MINIDUMP_CALLBACK_INFORMATION>(null).

[AArnott]

The friendly overload can encapsulate this though.