Disable type_checker_perf/slow/rdar23620262.swift since its flaky

Author: lplarson

validation-test/Sema/type_checker_perf/slow/rdar23620262.swift

@@ -1,6 +1,6 @@
 // RUN: %target-typecheck-verify-swift -solver-expression-time-threshold=1
 // REQUIRES: tools-release,no_asserts
-// XFAIL: rdar35803518
+// REQUIRES: disabled-rdar35803518
 
 let a: [Double] = []
 _ = a.map { $0 - 1.0 }