Disallow multiple 'default:' blocks in a single switch

Summary: Both we and PHP5 allow multiple default
statements and it's hard to argue that we're not
both kind of wrong in this behavior and should move to
only allowing one default statement per switch,
raising an error if multiple are found.

Ref: https://wiki.php.net/rfc/switch.default.multiple

Reviewed By: @JoelMarcey

Differential Revision: D1482354