[clang] [Darwin] Use macOS product version in ensureTargetInitialized (PR #206902)

[Paulius Velesko via cfe-commits]

pvelesko wrote:

@jhuber6 small follow-up to #183991 (which added `ensureTargetInitialized`): on macOS it recorded the Darwin *kernel* version from `getOSVersion()` instead of the macOS *product* version, so the later `AddDeploymentTarget()` call mismatches and trips the `setTarget` "Target already initialized!" assertion on the HIP-SPIR-V offload path. Would you mind reviewing? Thanks!

https://github.com/llvm/llvm-project/pull/206902