Marshalled arrays should include SizeParamIndex and direction

[AArnott]

For #269 we fixed a bug by emitting [MarshalAs(UnmanagedType.LPArray)] on parameters that carry arrays of COM objects.

We might further improve perf and function by adding the [Out] or [In] attributes from the metadata to these same parameters so the interop marshaler can skip some work.
We might also be useful to set the SizeParamIndex argument when the metadata includes it.

@skippy10110 mentioned in #333 (comment)_ a code snippet that included these elements that seem to still be missing.

In my case the [Out] is required for the call to work correctly. I don't segfault without it, but the array won't be populated if it's not included. (i.e. pceltFetched gets set to 1, but array[0] still equals null)

SizeParamIndex = 0 I'm able to remove without affecting anything.

I didn't look at putting together a minimal repro project for you after you linked the #269 issue. I can still do that if it would be useful.

[AArnott]

I don't think we need the repro any more now that we understand the fix. And thank you for helping me see the known runtime impact of omitting the [Out] attribute, as that raises this priority from "nice to have" to "must have".

[pziezio]

SizeParamIndex = 0 I'm able to remove without affecting anything.

In the [In] direction MarshalAsAttribute.SizeParamIndex documentation says that It does not have any effect on managed code that calls COM objects. That makes sense for me since we still need to pass pointer to array and size separately as specified in the COM interface.

In the [Out] direction Default Marshaling for Arrays says When marshaling arrays from unmanaged code to managed code, the marshaler checks the MarshalAsAttribute associated with the parameter to determine the array size. If the array size is not specified, only one element is marshaled. So if the new array is created by the unmanaged code the SizeParamIndex or SizeConst attributes are obligatory. That also makes sense since Marhaller has to know how many elements needs to allocate on the managed side.