Make platform element perform similar to feature element for diagnostic/quick fixes

[cherylking]

For a feature element, a quick fix is provided when the feature version does not match an known feature.

For the platform element, there is just a diagnostic and no quick fix.

I think we should provide a quick fix for the platform element in this case, showing all the possible choices for that platform. Also, it should not show a description for the platform element that is not a valid platform.