codegen: add == -> 'Double_Equal' to specialCharReplacements

[luketudge]

The double equal '==' is a common operator in a few contexts. I am using it for haystack operators, but there is also Python, Javascript, etc.

Currently if == appears in an enum its name gets sanitized to empty and generates invalid syntax. This problem is very similar to the one fixed in #12801 , so I am suggesting the same fix here.

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