[test] Re-enable test SILOptimizer/swap_refcnt.swift on non-Linux platforms. (swiftlang#7026)

Author: gparker42

test/SILOptimizer/swap_refcnt.swift

@@ -1,5 +1,7 @@
 // RUN: %target-swift-frontend -O -emit-sil %s | %FileCheck %s
-// REQUIRES: rdar:30181104 SILOptimizer/swap_refcnt.swift fails on linux.
+
+// FIXME: rdar://30181104 SILOptimizer/swap_refcnt.swift fails on linux.
+// UNSUPPORTED: OS=linux-gnu
 
 // Make sure we can swap two values in an array without retaining anything.