Add ABI baseline and checking for _Concurrency library on macOS

Add the generated ABI baseline for the _Concurency model on x86_64
macOS, along with a test that checks the built _Concurrency model
against that baseline.

Right now, the test is a mess, because we have some ABI breaks to
unwind. Add it so the test passes, and then we'll audit and address
every change.

Fixes rdar://83673282.

Author: DougGregor