[stdlib] Keep a StringStorage alive.

Address a stdlib lifetime issue exposed by SSADestroyHoisting's
more frequent (relative to DestroyHoisting's) expansion of store
[assign]s.

Author: nate-chandler

stdlib/public/core/StringGutsRangeReplaceable.swift

@@ -243,11 +243,13 @@ extension _StringGuts {
   internal mutating func appendInPlace(
     _ other: UnsafeBufferPointer<UInt8>, isASCII: Bool
   ) {
+    withExtendedLifetime(self._object) {
     self._object.nativeStorage.appendInPlace(other, isASCII: isASCII)
 
     // We re-initialize from the modified storage to pick up new count, flags,
     // etc.
     self = _StringGuts(self._object.nativeStorage)
+    }
   }
 
   @inline(never) // slow-path