Karoy Lorentey 73f349cb15 [stdlib] Rework String breadcrumbs initialization/loading ...

This is a wild guess at what might be causing our persistent, random
String failures on the main branch:

```
  Swift(macosx-x86_64) :: Prototypes/CollectionTransformers.swift
  Swift(macosx-x86_64) :: stdlib/NSSlowString.swift
  Swift(macosx-x86_64) :: stdlib/NSStringAPI.swift
  Swift(macosx-x86_64) :: stdlib/StringIndex.swift
  Swift-validation(macosx-x86_64) :: stdlib/String.swift
  Swift-validation(macosx-x86_64) :: stdlib/StringBreadcrumbs.swift
  Swift-validation(macosx-x86_64) :: stdlib/StringUTF8.swift
```

FWIW, it appears this is *not* caused by https://github.com/apple/swift/pull/62717:
that change has also landed on release/5.8, and I haven’t seen these
issues on that branch.

Our atomic breadcrumbs initialization vs its non-atomic loading
gives me an uneasy feeling that this may in fact be a long standing
synchronization issue that is only now causing problems (for whatever
reason). I am unable to reproduce these issues locally, so this guess
may be (and probably is) wildly off the mark, but this PR is likely
to be a good idea anyway, if only to rule out this possibility.

rdar://104751936

2023-02-10 20:23:56 -08:00

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