[test] Disable editor_placeholder_exec.swift for back deployment

Author: hamishknight

test/IDE/editor_placeholder_exec.swift

@@ -3,5 +3,9 @@
 // RUN: %FileCheck %s < %t/output
 // REQUIRES: executable_test
 
+// The runtime error format changed after the 5.3 release.
+// UNSUPPORTED: use_os_stdlib
+// UNSUPPORTED: back_deployment_runtime
+
 print(<#placeholder#>)
 // CHECK: editor_placeholder_exec.swift:[[@LINE-1]]: Fatal error: attempt to evaluate editor placeholder