[stdlib] Add String.subscript(_: ClosedRange<Index>) (#2653)

Fixes [SR-1596](https://bugs.swift.org/browse/SR-1596).

Author: moiseev

stdlib/public/core/StringRangeReplaceableCollection.swift.gyb

@@ -84,6 +84,14 @@ extension String {
   public subscript(bounds: Range<Index>) -> String {
     return String(characters[bounds])
   }
+
+  /// Accesses the text in the given range.
+  ///
+  /// - Complexity: O(*n*) if the underlying string is bridged from
+  ///   Objective-C, where *n* is the length of the string; otherwise, O(1).
+  public subscript(bounds: ClosedRange<Index>) -> String {
+    return String(characters[bounds])
+  }
 }
 
 public func == (lhs: String.Index, rhs: String.Index) -> Bool {

validation-test/stdlib/String.swift

@@ -205,6 +205,22 @@ StringTests.test("ForeignIndexes/subscript(Index)/OutOfBoundsTrap") {
   acceptor[i]
 }
 
+StringTests.test("String/subscript(_:Range)") {
+  let s = "foobar"
+  let from = s.startIndex
+  let to = s.index(before: s.endIndex)
+  let actual = s[from..<to]
+  expectEqual("fooba", actual)
+}
+
+StringTests.test("String/subscript(_:ClosedRange)") {
+  let s = "foobar"
+  let from = s.startIndex
+  let to = s.index(before: s.endIndex)
+  let actual = s[from...to]
+  expectEqual(s, actual)
+}
+
 StringTests.test("ForeignIndexes/subscript(Range)/OutOfBoundsTrap/1") {
   let donor = "abcdef"
   let acceptor = "uvw"