Using XFAIL won't work here because the tests pass on the iOS back deployment bots, resulting in a UPASS.
I was able to take a look at the test binary and rule out both codegen for `if #available(...)` and the optimizer as culprits so the next step is to check if `_stdlib_isOSVersionAtLeast()` is actually busted in the runtime libraries in older versions of macOS.
* [SR-14424] Add 'moveInitialize' test case for small string 'init(unsafeUninitializedCapacity:initializingUTF8With:)'
...and make an adjustment to an existing test case.
* [stdlib][test] Update test cases for small string unsafeUninitializedCapacity based on reviewer feedback.
* [fixup]
