[marshal] Fix VARIANT marshaling in structs (#20917)

* [tests] Add BStr and VARIANT in structs interop tests

* [marshal] Fix reverse P/Invoke marshaling VARIANT fields

Partial revert of https://github.com/mono/mono/pull/8732

If a managed struct is declared as

        public struct StructWithVariant
        {
            [MarshalAs (UnmanagedType.Struct)]
            public object data;
        };

Then the `data` field should be marshalled as a VARIANT.

The fix checks that the field's type is `object` and uses VARIANT for
marshaling, otherwise it continues as in mono/mono#8732 and uses
MONO_MARSHAL_CONV_OBJECT_STRUCT

0 files changed, 0 insertions, 0 deletions