[clang] [clang][Darwin] Prefer the toolchain-provided libc++.dylib if there is one (PR #170303)

[Tom Stellard via cfe-commits]

https://github.com/tstellar commented:

Have you tested this with a multi-target runtime build?  My fix was slightly different so maybe you won't have the same problem, but I ran into an issue where clang would pick up the libc++ build for watchos instead of the one build for macOS.  Here is an example of the error: https://github.com/llvm/llvm-project/actions/runs/18506722806/job/52742884609?pr=163026

https://github.com/llvm/llvm-project/pull/170303