Use withUnsafeTemporaryAllocation instead of a temporary Array

stdlib/public/core/UnicodeHelpers.swift

@@ -396,17 +396,15 @@ extension _StringGuts {
       ).0))
     }
 
-    // TODO(String performance): Stack buffer if small enough
-    let cus = Array<UInt16>(unsafeUninitializedCapacity: count) {
-      buffer, initializedCapacity in
+    return withUnsafeTemporaryAllocation(
+      of: UInt16.self, capacity: count
+    ) { buffer in
       _cocoaStringCopyCharacters(
         from: self._object.cocoaObject,
         range: start..<end,
-        into: buffer.baseAddress._unsafelyUnwrappedUnchecked)
-      initializedCapacity = count
-    }
-    return cus.withUnsafeBufferPointer {
-      return Character(String._uncheckedFromUTF16($0))
+        into: buffer.baseAddress._unsafelyUnwrappedUnchecked
+      )
+      return Character(String._uncheckedFromUTF16(UnsafeBufferPointer(buffer)))
     }
 #else
     fatalError("No foreign strings on Linux in this version of Swift")