[swift-evolution] Remove nil and NilLiteralConvertible

[E. Maloney]

> On Jun 8, 2016, at 4:59 PM, Brandon Knope via swift-evolution <swift-evolution at swift.org> wrote:
> 
> Is it really an implementation detail? It is very leaky if it is one because it is highly exposed to everyone.

It’s only as leaky as you want it to be. We have two shipping apps with large codebases using Swift, and we *never* refer to Optional enum directly or to its enum .some or .none cases anywhere in our code.

Personally, I’d much prefer to keep nil.