The macro simultaneously declares an enumeration and a count of its
enumerators.
A few variants of the macro are also provided to handle things like
enum classes, underlying types, and enumerations declared at class
scope.
MozReview-Commit-ID: 3z6yHnfXbLj
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extra : rebase_source : 92c333693e4bbf85b89cd3d7ac5b31f4b5434367
