Auto-generate our SxS manifest

I found a todo in the code that I don't think was ever ported to github.
```c++
TODO MSFT#21300206: Switch to automatically generating these.
```

I believe @DHowett-MSFT was working on this before we went open-source, but `mt.exe` wasn't updated to handle our scenario at the time. Not sure if that still applies.

This might affect some of our elevated launch bugs IIRC.