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

[Louis Dionne via llvm-commits]

ldionne wrote:

I ended up doing a full revert of this patch due to failures in LLDB tests that will take longer to address.

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