[test] Require OS X for an ASan test

ASan doesn't have driver support on Linux.

Author: vedantk

test/IRGen/coverage_with_asan.swift

@@ -1,4 +1,5 @@
 // RUN: %target-swift-frontend %s -profile-generate -profile-coverage-mapping -sanitize=address -emit-ir -o - | FileCheck %s
+// REQUIRES: OS=macosx
 
 // CHECK: main
 func main() {}