[Test] Mark 3 tests unsupported on use_os_stdlib.

These tests require an OS with Swift 5.9, and that can't be expressed as
a REQUIRES: line.

rdar://114564735

Author: nate-chandler

test/Interpreter/layout_string_witnesses_dynamic.swift

@@ -10,6 +10,8 @@
 
 // REQUIRES: executable_test
 
+// Requires runtime functions added in Swift 5.9.
+// UNSUPPORTED: use_os_stdlib
 // UNSUPPORTED: back_deployment_runtime
 
 import Swift

test/Interpreter/layout_string_witnesses_objc.swift

@@ -11,6 +11,9 @@
 // REQUIRES: executable_test
 // REQUIRES: objc_interop
 
+// Requires runtime functions added in Swift 5.9.
+// UNSUPPORTED: use_os_stdlib
+
 import Swift
 import layout_string_witnesses_types
 import ObjCClasses

test/Interpreter/layout_string_witnesses_static.swift

@@ -12,6 +12,8 @@
 
 // REQUIRES: executable_test
 
+// Requires runtime functions added in Swift 5.9.
+// UNSUPPORTED: use_os_stdlib
 // UNSUPPORTED: back_deployment_runtime
 
 import Swift