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

[Ian Anderson via cfe-commits]

ian-twilightcoder wrote:

It's a problem for the dylibs because you'll end up with a broken executable for rpath reasons. I don't think it makes sense to add XcodeDefault.xctoolchain/usr/lib to everyone's search paths by default. There's nothing in there that anyone should use outside of Xcode, and things that get packaged inside of Xcode. Maybe @cachemeifyoucan or https://github.com/jakepetroules have other opinions.

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