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

[Louis Dionne via cfe-commits]

ldionne wrote:

Thanks for the reviews.

For information, I'm currently investigating some new failures I'm seeing when doing a bootstrapping build. The compiler-rt build is complaining about something new I wasn't seeing before (but I did a clean reinstall of my system in between, so some stuff may have changed). Before I land this, I'll also want to make sure I understand the RPATH story for the toolchain we're building upstream -- since it's the first time we'll actually be linking against the `libc++.dylib` that we build in the toolchain, that is something new we haven't tested yet and I want to make sure everything behaves correctly.

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