test: fix some warnings in closure-multivalue test

Change `var` to let as per the warning.  NFC.

Author: compnerd

test/DebugInfo/closure-multivalue.swift

@@ -20,8 +20,8 @@ public func sort(_ a: String, b: String) -> Bool {
 }
 
 public func demo() {
-    var names = ["Sean", "Barry", "Kate"]
-    var sortedNames = names.sorted(isOrderedBefore: sort)
+    let names = ["Sean", "Barry", "Kate"]
+    let sortedNames = names.sorted(isOrderedBefore: sort)
     var sortedNamesAsString : String = String()
     for name in sortedNames {
         sortedNamesAsString += ("\(name), ")