Addressed review comments.

Author: David Ungar

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraph.swift

@@ -821,7 +821,7 @@ extension ModuleDependencyGraph {
           return Int(u)
         }
         func internedString(field i: Int) throws -> InternedString {
-          try InternedString(index: stringIndex(field: i))
+          try InternedString(deserializedIndex: stringIndex(field: i))
         }
         func nonemptyInternedString(field i: Int) throws -> InternedString? {
           let s = try internedString(field: i)
@@ -988,6 +988,12 @@ extension ModuleDependencyGraph {
   }
 }
 
+fileprivate extension InternedString {
+  init(deserializedIndex: Int) {
+    self.index = deserializedIndex
+  }
+}
+
 extension ModuleDependencyGraph {
   /// Attempts to serialize this dependency graph and write its contents
   /// to the given file path.

Sources/SwiftDriver/IncrementalCompilation/ModuleDependencyGraphParts/InternedStrings.swift

@@ -24,14 +24,12 @@ public struct InternedString: CustomStringConvertible, Equatable, Hashable {
     self.init(index: s.isEmpty ? 0 : table.intern(s))
   }
 
-  public var isEmpty: Bool { index == 0 }
-  
-  // This constructor cannot be private because it is called from the `internedString`
-  // method in ``ModuleDependencyGraph/deserialize/Visitor``.
-  init(index: Int) {
+  private init(index: Int) {
     self.index = index
   }
 
+  public var isEmpty: Bool { index == 0 }
+    
   public static var empty: Self {
     let r = Self(index: 0)
     assert(r.isEmpty)