[stdlib] Drop unneeded protocol requirement

Conversion to cocoa is not a common requirement for all models of
_ArrayBufferType

Swift SVN r22947

test/1_stdlib/NewArray.swift.gyb

@@ -312,7 +312,7 @@ func testSlice() {
 
   // Now a bridged slice
   var a = Array<NSString>(
-    _ArrayBuffer(nsArrayOfStrings()._asCocoaArray()))
+    _ArrayBuffer(nsArrayOfStrings()._buffer._asCocoaArray()))
   
   printSequence(a)                // CHECK-NEXT: [foo, bar, baz]