Swift: update test output

The 'first' field is seen as a TaintInheritingContent, which means any read step for 'first' becomes a taint step too.
This type of taint step does not permit an implicit read before it, because it wasn't contributed by a configuration.
So there is no way for the taint to get out of the collection content before the taint step through '.first'.
The test previously passed because an implicit read at once of the earlier sinks could follow use-use flow down to the receiver of .first,
allowing it to escape the collection content.

Author: asgerf

swift/ql/test/library-tests/dataflow/taint/libraries/set.swift

@@ -28,7 +28,7 @@ func testSet(ix: Int) {
   sink(arg: taintedSet.max()!) // $ tainted=t1
   sink(arg: taintedSet.firstIndex(of: source("t2"))!)
   sink(arg: taintedSet[taintedSet.firstIndex(of: source("t3"))!]) // $ tainted=t1
-  sink(arg: taintedSet.first!) // $ tainted=t1
+  sink(arg: taintedSet.first!) // $ MISSING: tainted=t1
   for elem in taintedSet {
     sink(arg: elem) // $ tainted=t1
   }
@@ -100,7 +100,7 @@ func testSet(ix: Int) {
   sink(arg: taintedSet.sorted().randomElement()!) // $ tainted=t1
   sink(arg: taintedSet.shuffled().randomElement()!) // $ tainted=t1
 
-  sink(arg: taintedSet.lazy[taintedSet.firstIndex(of: source("t11"))!]) // $ tainted=t1
+  sink(arg: taintedSet.lazy[taintedSet.firstIndex(of: source("t11"))!]) // $ MISSING: tainted=t1
 
   var it = taintedSet.makeIterator()
   sink(arg: it.next()!) // $ tainted=t1