Port CustomMarshalers.dll to C# and .NET Core (#21343)

Port CustomMarshalers.dll to C# and .NET Core (#21343)

* Import CustomMarshalers from corefx PR.

* Add tests for marshalling IEnumerator and IEnumerable using the System.Runtime.InteropServices.CustomMarshalers.

* Verify that the marshaller is used in the managed -> native direction in the tests.

* Remove #if's

* PR Feedback.

* Move ComHelpers.h into interop common headers.

* Add some template machinery to ensure that we don't accidentally pass in an incorrect type into DoQueryInterface.

* Add VariantClear

* Make marshalers internal and root them in ILLinkTrim.xml

* Revert enable-if machinery.

* Add tests for the pass-through unwrapping done by the EnumeratorToEnumVariantMarshaler.

* Remove some of my helper code.

* Added todo about exposing EnumerableToDispatchMarshaler.

* PR Feedback.

* Removed the last placement new.

* Use the runtime hooks to enable us to not have to publically expose the custom marshalers.

* Remove out of date comments.