Roslyn: Marshalling methods with `in` parameters should have [In] attribute

[OmarTawfik]

As an optimization, compiler should place [InAttribute] if user didn't on in parameters in marshaling methods. This saves an extra copy by the run-time on method return. because right now, it is treated as an ordinary ref, and the value is copied in and out.

cc @tannergooding @jcouv

[tannergooding]

Related to https://github.com/dotnet/coreclr/issues/11830 and https://github.com/dotnet/csharplang/issues/38#issuecomment-303570195

FYI. @yizhang82

[jcouv]

:memo: It's ok to fix this after 7.2, as it is an optimization. Users can manually add the [In] as a workaround (when the compiler will add it, it will be superfluous, but won't break).

[jcouv]

@OmarTawfik Can you take this one?