[stdlib] String: Avoid retain/release operations around use sites of sharedStorage and cocoaObject

stdlib/public/core/StringGuts.swift

@@ -249,12 +249,11 @@ extension _StringGuts {
   ) -> Int? {
     #if _runtime(_ObjC)
     // Currently, foreign  means NSString
-    if let res = _cocoaStringCopyUTF8(_object.cocoaObject,
-      into: UnsafeMutableRawBufferPointer(start: mbp.baseAddress,
-                                          count: mbp.count)) {
-      return res
+    let res = _object.withCocoaObject {
+      _cocoaStringCopyUTF8($0, into: UnsafeMutableRawBufferPointer(mbp))
     }
-    
+    if let res { return res }
+
     // If the NSString contains invalid UTF8 (e.g. unpaired surrogates), we
     // can get nil from cocoaStringCopyUTF8 in situations where a character by
     // character loop would get something more useful like repaired contents