https://github.com/swiftlang/swift/pull/37774 added '-clang-target' which allows us to specify a target triple that only differs from '-target' by the OS version, when we want to provide a different OS version for API availability and type-checking, in order to set a common/unified target triple for the entire Clang module dependency graph, for presenting a unified API surface to the Swift client, serving as a maximum type-checking epoch.
This change adds an equivalent flag for the '-target-variant' configuration, as a mechanism to ensure that the entire module dependency graph presents a consistent os version.
