THRIFT-5150: Fix compilation with Swift 5.2

- (OS X and Linux) type 'TSet<Element>' does not conform to protocol 'Collection'
- (Linux only) kCFSocketCloseOnInvalidate has Int type

Client: swift
diff --git a/lib/swift/Sources/TSet.swift b/lib/swift/Sources/TSet.swift
index d340fec..9d89d14 100644
--- a/lib/swift/Sources/TSet.swift
+++ b/lib/swift/Sources/TSet.swift
@@ -30,6 +30,7 @@
   
   /// Mark: Collection
   
+  public typealias Element = Storage.Element
   public typealias Indices = Storage.Indices
   public typealias Index = Storage.Index
   public typealias IndexDistance = Int