Enable Requirement Machine unconditionally for test/Prototypes/BigInt.swift

This would fail in asserts builds because the GSB produced an invalid
minimal signature. It now started failing in noassert builds as well
because we cross-check generic signatures against the Requirement
Machine.

Instead run this test with the Requirement Machine enabled unconditionally
since it works now.

Author: slavapestov

test/Prototypes/BigInt-windows.swift

@@ -11,17 +11,12 @@
 //===----------------------------------------------------------------------===//
 
 // RUN: %empty-directory(%t)
-// RUN: %target-build-swift -swift-version 4 -o %t/a.out %s
+// RUN: %target-build-swift -swift-version 4 -o %t/a.out %s -Xfrontend -requirement-machine-inferred-signatures=on -Xfrontend -requirement-machine-abstract-signatures=on -Xfrontend -requirement-machine-protocol-signatures=on
 // RUN: %target-codesign %t/a.out
 // RUN: %target-run %t/a.out
 // REQUIRES: executable_test
 // REQUIRES: CPU=x86_64
 
-// See rdar://problem/65251059
-// UNSUPPORTED: windows
-// rdar://problem/65015626
-// XFAIL: asserts
-
 import StdlibUnittest
 #if canImport(Darwin)
   import Darwin