[MAYA-123329] DX11shader not saved in USD file, mesh will have no "Looks" associated with it.

**Describe the bug**
If a mesh object has a "dx11shader" applied to it, when exporting a USD file there will be no material or shader saved.

**Steps to reproduce**
1. Open attached scene.
2. Export All, choose a filename.
3. Open the file in "usdview", notice there is no material on the sphere.

**Expected behavior**
I would expect at least the correct material name to be saved in the USD file. I understand that it couldn't reliably gather textures since it's dependent on an fx file, at the very least we'd expect the correct material name to be included.

**Attachments**
[dx11_test.zip](https://github.com/Autodesk/maya-usd/files/4847569/dx11_test.zip)

**Specs (if applicable):**
Windows 10
Maya PR 115