[clang] [compiler-rt] Revert "[clang][Darwin] Prefer the toolchain-provided libc++.dylib if there is one" (PR #170912)

[Louis Dionne via llvm-commits]

ldionne wrote:

LMK if you're fine with this way of fixing the issue (temporarily) instead: it will allow me to investigate while unbreaking the bots, and that way we get additional feedback on this change since we don't revert entirely.

https://github.com/llvm/llvm-project/pull/170912