Field/Property assigned via await is not detected when field/property is only an IDisposable

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Prerequisites

• [x] I have written a descriptive issue title
• [x] I have verified that I am running the latest version of DisposableFixer: 1.6.0
• [x] I have searched open and closed issues to ensure it has not already been reported

Description

When I assigned an MemoryStream via await to a field that is an not disposed IDisposable, this is not detected.

Source Code

namespace SomeNamespace
{
    public class SomeClass
    {
        private IDisposable _mem;

        public async Task Do()
        {
            _mem = await Create();
        }
        public Task<MemoryStream> Create()
        {
            return Task.FromResult(new MemoryStream());
        }
    }
}

Screenshot

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the problem is, that GetAwaitExpressionInfo returns null, This might by a problem in roslyn fixed in version 3.x. But in order to upgrade to these, first we have to migrate to .Net Standard 2.0

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Blocked by #126

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Same result with Roslyn 3.3.1