The `@exclusivity(unchecked)` attribute can be used on variables to selectively disable exclusivity checking.
For completeness, also the `@exclusivity(checked)` variant is supported: it turns on exclusivity checking for specific variables if exclusivity enforcement is disabled by the command line option.
This new attribute is a missing implementation part of SE-0176 (https://github.com/apple/swift-evolution/blob/main/proposals/0176-enforce-exclusive-access-to-memory.md).
rdar://31121356
