Revert "[cmake] Ignore required explicit availability in Swift stdlib"
Revert "[cmake] Ignore required explicit availability in Swift stdlib"

This reverts commit 4bec9d54e72cc13858b1eaae9abb12eed7e93c81.

apple/swift,roambotics/swift,roambotics/swift,roambotics/swift,apple/swift,apple/swift,apple/swift,roambotics/swift,apple/swift,apple/swift,roambotics/swift,roambotics/swift