Replace stdlib and test/stdlib 9999 availability.

[stephentyrone]

macOS 9999 -> macOS 10.15
iOS 9999 -> iOS 13
tvOS 9999 -> tvOS 13
watchOS 9999 -> watchOS 6

Less churn-inducing reworking of #25230

[stephentyrone]

Note that this does not include changes to test/*, only test/stdlib/*. There will be a follow-on PR with the remaining test changes. This also does not change the availability for MathFunctions, because we've pended that out of 5.1.

[stephentyrone]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - a8a590a

[stephentyrone]

@milseman This is failing on a guard that you added in b19c2cf9c33:

  private init?<S: StringProtocol>(
    _ range: NSRange, _genericIn string: __shared S
  ) {
    // Corresponding stdlib version
    guard #available(macOS 10.15, iOS 13, tvOS 13, watchOS 6, *) else {
      fatalError()
    }
    ...
  }

What is this check actually trying to do? This init is called unconditionally from

  public init?(_ range: NSRange, in string: __shared String) {
    self.init(range, _genericIn: string)
  }

so I don't see how it was ever going to do the right thing.

[stephentyrone]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - a8a590a

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - a8a590a

[milseman]

LGTM, other than being dead code that we're adding to service an incomplete availability model.