Merge pull request #62891 from lorentey/reenable-string-index-test

[test] Reenable stdlib/StringIndex, requiring optimized_stdlib

Author: lorentey

test/stdlib/StringIndex.swift

@@ -7,11 +7,9 @@
 // 5.7 so that we can test new behavior even if the SDK we're using predates it.
 
 // REQUIRES: executable_test
+// REQUIRES: optimized_stdlib
 // UNSUPPORTED: freestanding
 
-// Temporarily disabled due to extremely slow execution in the "UTF-16 breadcrumbs" test
-// REQUIRES: rdar103934958
-
 import StdlibUnittest
 #if _runtime(_ObjC)
 import Foundation