[test] Restore test and remove old portion

Author: milseman

test/stdlib/UnavailableStringAPIs.swift.gyb

@@ -2,8 +2,6 @@
 // RUN: %gyb -D_runtime=%target-runtime %s -o %t/main.swift
 // RUN: %line-directive %t/main.swift -- %target-swift-frontend -typecheck -verify %t/main.swift
 
-// REQUIRES: rdar44572521
-
 func test_StringSubscriptByInt(
   x: String,
   i: Int,
@@ -15,13 +13,6 @@ func test_StringSubscriptByInt(
   _ = x[r2] // expected-error {{'subscript(_:)' is unavailable: cannot subscript String with an integer range, see the documentation comment for discussion}} {{none}}
 }
 
-% if _runtime == 'objc':
-func test_UTF16ViewSubscriptByInt(x: String.UTF16View, i: Int, r: Range<Int>) {
-  _ = x[i] // expected-error {{'subscript(_:)' is unavailable: Indexing a String's UTF16View requires a String.UTF16View.Index, which can be constructed from Int when Foundation is imported}} {{none}}
-  _ = x[r] // expected-error {{'subscript(_:)' is unavailable: Slicing a String's UTF16View requires a Range<String.UTF16View.Index>, String.UTF16View.Index can be constructed from Int when Foundation is imported}} {{none}}
-}
-% end
-
 func test_UTF8View(s: String.UTF8View, i: String.UTF8View.Index, d: Int) {
   _ = s.index(after: i) // OK
   _ = s.index(before: i) // OK