`@_implementationOnly import _AtomicsShims` triggers compiler assertions on 5.9

[lorentey]

Under certain unknown circumstances, using Swift Atomics as a dependency triggers a compiler assertion due to the way it imports _AtomicsShims in 5.9.

error: fatal error encountered while reading from module '[...]'; please submit a bug report (https://swift.org/contributing/#reporting-bugs)
<binary format>
^
Please submit a bug report (https://swift.org/contributing/#reporting-bugs) and include the crash backtrace.
Stack dump:
[...]
6.      *** DESERIALIZATION FAILURE ***
[...]
module '_AtomicsShims' was not loaded

[lorentey]

I added the @_implementationOnly import to prevent issues when library evolution is enabled:

#104 (comment)

Not sure what we can do here, other than getting rid of this C module as soon as possible.