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
When the stdlib says not to check, it’s a good idea to actually not have any checking, so that we leave existing code paths unchanged. (And because we do trust that the stdlib is responsible about such things.)
