[Build][Backtracing] Make sure the target for OS X is at least 10.14.4.

[al45tair]

We need to make sure that we build swift-backtrace with a deployment target newer than 10.14.4 in order that we get linked against /usr/lib/swift/libswiftCore.dylib rather than using an @rpath-based path.

If we fail to do that, dyld becomes confused and we end up crashing with weird errors about missing method implementations on Swift.__StringStorage.

To make this work, add support for DEPLOYMENT_VERSION_* in the add_swift_target_executable() CMake function. And since I spotted a bug in it, fix the existing support in add_swift_target_library() while I'm there.

rdar://132710670

[al45tair]

@swift-ci Please test

[al45tair]

Note: this doesn't affect the overall correctness of the final build. It just fixes problems with the tests.

[al45tair]

Explanation: If the deployment target is set to anything prior to 10.14.4, the $ld$previous feature is used to replace the path to the Swift libraries with an @rpath-based path. This is done to permit Swift programs to be back-deployed to systems where Swift was not part of the OS. Unfortunately, doing this breaks the use of DYLD_LIBRARY_PATH in the tests to point things at the newly built libswiftCore.dylib, because dyld looks at the original load path to determine whether or not to override the system copy of the library, and if it sees @rpath on the front, it assumes that it doesn't need to do that, which means that other system libraries still have pointers to the system copy of libswiftCore.dylib, which then results in sadness.
Risk: Only affects tests. The final binaries are built with a different deployment target rather than the default 10.13.
Original PR: #75709
Reviewed by: @edymtt
Resolves: rdar://132710670
Tests: Backtracing/EarlyMessage.swift and Backtracing/Timing.swift will fail for some builds without this change. With it, they work.

[al45tair]

Sounds like we aren't going to fix this for release 6.0. It only affects the tests anyway (the actual build will be done with an appropriate target).