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

[Kewen Meng via cfe-commits]

Kewen12 wrote:

Hi! It seems this PR breaks bots. Could you please take a look? Thanks!

https://lab.llvm.org/buildbot/#/builders/10/builds/18578

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