[libc++] Allow backdeployment CI to run on newer macOS

This should work now that we are using a matching libunwind.dylib when
we run the tests in back-deployment scenarios. The only restriction we
have now is to run on macOS x86_64, since that's what the old dylibs
were compiled for. This should allow us to move to newer AppleClangs
in the CI.

As a fly-by, fix missing availability annotations on optional's
monadic operations.

Differential Revision: https://reviews.llvm.org/D119840