[concurrency] new Clock and Executor symbol availability fix

[glessard]

After we (finally) merged the updated the availability macros for SwiftStdlib 6.2, some new Clock- and executor-related symbols had the wrong availability.

This is a simple attempt at a fix.

Addresses rdar://162819688

[glessard]

@swift-ci please smoke test

[ktoso]

This looks right to me but I'd like @al45tair to chime in what the plan was here about the availability of those. The 6.2 seems accidental, but perhaps I'm missing something.