The location of the Flow enum bodys was incorrect. I fixed this in flow-parser (facebook/flow@498a209), and this PR fixes this in Babel, making it consistent with flow-parser, and also consistent with what the enum body contains (everything after enum E).

The diff also does some minor refactoring and cleaning up:

• Splits out parsing of the enum body into a separate function (separate from parsing the enum declaration overall)
• Rather than have a helper function for enum string bodies, it returns the string members, so the creation of enum bodies is completely contained in the new flowEnumBody function.

This incorrect location was discovering when adding support in Prettier prettier/prettier#6860

After this lands and they update their parser deps, Prettier can remove their hacks.