[TypeChecker] NFC: Enable Swift -> C implicit pointer conversions tests only on Darwin

Author: xedin

test/Constraints/swift_to_c_pointer_conversions.swift.gyb

@@ -6,6 +6,8 @@
 
 // RUN: %line-directive %t/swift_to_c_pointer_conversions.swift -- %target-swift-frontend(mock-sdk: %clang-importer-sdk-nosource -I %t) -import-objc-header %S/Inputs/c_pointer_conversions.h %t/swift_to_c_pointer_conversions.swift -typecheck -verify
 
+// REQUIRES: objc_interop
+
 func test_raw_ptr(ptr: UnsafeRawPointer) {
   char_ptr_func(ptr) // expected-error {{}}
   unsigned_char_ptr_func(ptr) // expected-error {{}}