[WIP] Make enum sealed even without x-ms-enum

[timotheeguerin]

fix Azure/autorest#3797

Todo:

• Verify this is not going to cause any unexpected result on existing specs
• Tests

[azure-pipelines]

You may test this build by running autorest --reset and then either:

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add --use: to the command line:

autorest --use:https://tinyurl.com/y2sgapro ...

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or use the following in your autorest configuration:

use-extension:
  "@autorest/modelerfour": "https://tinyurl.com/y2sgapro" 

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If this build is good for you, give this comment a thumbs up. (👍)

And you should run autorest --reset again once you're finished testing to remove it.