DRY some methods in IPAddress
DRY some methods in IPAddress

Summary:
[Folly] DRY some methods in `IPAddress`.

Specifically, the ones which just delegate to either `asV4()` or to `asV6()`.

Reviewed By: mzlee

Differential Revision: D5529797

fbshipit-source-id: 4dd3dc893ab19281325700b85400c1c1aadfd77f

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