[4.2] Disable test/SILOptimizer/unused_containers.swift

It is succeeding unexpectely on bots. Disable until somebody gets a chance to look at this

rdar://39002151

Author: aschwaighofer

test/SILOptimizer/unused_containers.swift

@@ -2,6 +2,9 @@
 
 // REQUIRES: swift_stdlib_no_asserts
 
+// This is failing bots.
+// REQUIRES: rdar39002151
+
 // The below temporary disables this XFAIL-ed test on Linux for
 // https://github.com/apple/swift/pull/14755, where it unexpectedly passes.
 // @gottesmm is working on a fix.