[Test] Conditionalize KVOKeyPaths tests that only work with Swift 5.1

Some of the tests in KVOKeyPaths.swift check problems that were not fixed
until Swift 5.1 so they are not expected to pass when running with the
5.0 libraries.

rdar://problem/50173830

Author: bob-wilson

test/stdlib/KVOKeyPaths.swift

@@ -1,4 +1,8 @@
-// RUN: %target-run-simple-swift | %FileCheck %s
+// RUN: %empty-directory(%t)
+// RUN: %target-build-swift %s -o %t/a.out
+// RUN: %target-codesign %t/a.out
+// RUN: %target-run %t/a.out | grep 'check-prefix' > %t/prefix-option
+// RUN: %target-run %t/a.out | %FileCheck -check-prefix=CHECK `cat %t/prefix-option` %s
 // REQUIRES: executable_test
 
 // REQUIRES: objc_interop
@@ -117,6 +121,16 @@ print("target removed")
 // Test NSKeyValueObservingCustomization issue with observing from the callbacks
 //===----------------------------------------------------------------------===//
 
+// The following tests are only expected to pass when running with the
+// Swift 5.1 and later libraries.
+if #available(iOS 9999, macOS 9999, tvOS 9999, watchOS 9999, *) {
+  print("-check-prefix=CHECK-51")
+} else {
+  print("-check-prefix=DONT-CHECK")
+  // Need at least one check, otherwise FileCheck will complain.
+  // DONT-CHECK: {{.}}
+}
+
 class Target2 : NSObject, NSKeyValueObservingCustomization {
     @objc dynamic var name: String?
 
@@ -151,10 +165,10 @@ withExtendedLifetime(Target2()) { (target) in
 }
 print("observer removed")
 
-// CHECK-LABEL: registering observer for Target2
-// CHECK-DAG: keyPathsAffectingValue: key == \.name: true
-// CHECK-DAG: automaticallyNotifiesObservers: key == \.name: true
-// CHECK-NEXT: observer removed
+// CHECK-51-LABEL: registering observer for Target2
+// CHECK-51-DAG: keyPathsAffectingValue: key == \.name: true
+// CHECK-51-DAG: automaticallyNotifiesObservers: key == \.name: true
+// CHECK-51-NEXT: observer removed
 
 //===----------------------------------------------------------------------===//
 // Test NSSortDescriptor keyPath support
@@ -175,5 +189,5 @@ let descriptor = NSSortDescriptor(keyPath: \Sortable1.name, ascending: true)
 _ = NSSortDescriptor(keyPath: \Sortable2.name, ascending: true)
 print("keyPath == \\Sortable1.name:", descriptor.keyPath == \Sortable1.name)
 
-// CHECK-LABEL: creating NSSortDescriptor
-// CHECK-NEXT: keyPath == \Sortable1.name: true
+// CHECK-51-LABEL: creating NSSortDescriptor
+// CHECK-51-NEXT: keyPath == \Sortable1.name: true