TSCUtility: deprecate Platform

This marks the `Platform` type as deprecated.  The functionality has
been split up into swift-package-manager and sourcekit-lsp as
appropriate.  There remain no users of this at this point.

apple/sourcekit-lsp#686
apple/swift-package-manager#6011

Sources/TSCUtility/Platform.swift

@@ -12,6 +12,7 @@ import TSCBasic
 import Foundation
 
 /// Recognized Platform types.
+@available(*, deprecated, message: "moved to SourceKit-LSP")
 public enum Platform: Equatable {
     case android
     case darwin