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

[Louis Dionne via llvm-commits]

ldionne wrote:

Alright, I've tested with a slightly modified Homebrew formula (they change the installation directory of `libc++.dylib` and I think we have to undo that), and everything seems to be working. I'm comfortable with landing this now.

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