Allow default nil values for optional properties

[natecook1000]

This adds underscored initializers that let library users add = nil to declarations of optional @Option and @Argument properties. Previously, default values have been available for properties of non-optional types only.

These new initializers use _OptionalNilComparisonType as the wrapped value parameter, so only a nil literal is acceptable in the default value position. This avoids the problem of declaring an optional property with a non-nil default, which ends up negating the purpose of an optional.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary

[natecook1000]

@swift-ci Please test

[rauhul]

Just wondering, why is this desired? Are we expecting an upcoming language mode where var x: T? doesn't have a default value of nil? (I would love that change)