[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:

I would be tempted to disable the test on non-Darwin instead, while I figure out how to best re-enable it. All of the failures seem to be on Linux builders which happen to be using `lld`.

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