THRIFT-4902: Swift 4 and 5 compatibility (#1827)

diff --git a/lib/swift/Sources/TSet.swift b/lib/swift/Sources/TSet.swift
index 1ecd170..6891c11 100644
--- a/lib/swift/Sources/TSet.swift
+++ b/lib/swift/Sources/TSet.swift
@@ -32,7 +32,7 @@
   
   public typealias Indices = Storage.Indices
   public typealias Index = Storage.Index
-  public typealias IndexDistance = Storage.IndexDistance
+  public typealias IndexDistance = Int
   public typealias SubSequence = Storage.SubSequence