Tests: Add integration tests for @_backDeploy.

[tshortli]

Add integration tests for @_backDeploy designed to verify that use of declarations annotated with @_backDeploy behave as expected when running a client binary on an older OS that does not have the back deployed APIs. Verification is performed the following way:

1. Build a helper framework with the availability macro BackDeploy 1.0 defined to an OS version before Swift ABI stability and version 2.0 of that macro defined to an OS version after Swift ABI stability. Note that stradling ABI stability is not a necessary requirement of this test; it's just convenient to use OS versions that correspond to existing lit substitutions.
2. Build the client executable with a deployment target set to the same OS version as BackDeploy 2.0.
3. Run the client executable, verifying that the copies of the functions in the framework are used (verified by runtime logic that compares the results of #dsohandle).
4. Build a new copy of the helper framework, this time with BackDeploy 2.0 defined to a distant future OS version.
5. Build a new copy of the client executable against the new framework and the deployment target set to the same OS version as BackDeploy 1.0.
6. Run the new executable, verifying via #dsohandle that client copies of the APIs are used.
7. Re-build the framework in place, this time stripping out the definitions of the back deployed APIs entirely.
8. Re-run the unmodified executable, with the same expectations as in (6). However, this time we're also verifying that the executable can run even without the original API symbols present in the linked dylib.

[tshortli]

@swift-ci please test

[tshortli]

@swift-ci please test macOS