[stdlib] Make String.UTF8View bidirectional

This is a step along the way toward handling backward-compatiblity of UTF8View
slicing and preventing inadvertent creation of String instances that keep
inaccessible memory alive.

validation-test/stdlib/String.swift

@@ -59,10 +59,10 @@ StringTests.test("AssociatedTypes-UTF8View") {
   expectCollectionAssociatedTypes(
     collectionType: View.self,
     iteratorType: View.Iterator.self,
-    subSequenceType: Slice<View>.self,
+    subSequenceType: BidirectionalSlice<View>.self,
     indexType: View.Index.self,
     indexDistanceType: Int.self,
-    indicesType: DefaultIndices<View>.self)
+    indicesType: DefaultBidirectionalIndices<View>.self)
 }
 
 StringTests.test("AssociatedTypes-UTF16View") {

validation-test/stdlib/StringViews.swift

@@ -728,8 +728,7 @@ tests.test("String.UTF8View/Collection")
   .forEach(in: utfTests) {
   test in
 
-  // FIXME(ABI)#72 : should be `checkBidirectionalCollection`.
-  checkForwardCollection(test.utf8, test.string.utf8) { $0 == $1 }
+  checkBidirectionalCollection(test.utf8, test.string.utf8) { $0 == $1 }
 }
 
 tests.test("String.UTF16View/BidirectionalCollection")