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

[Louis Dionne via llvm-commits]

ldionne wrote:

Note that it's possible that landing this patch is going to expose places where we were previously using the SDK libc++ headers when we really intended to be using the toolchain libc++ headers. The failure mode for that would be that a program is compiled against SDK headers, and hence makes references to stuff that's only present in Apple's system `libc++.dylib`. But then, if the program ends up being linked against the toolchain's `libc++.dylib`, which might not contain the same definitions, we'd get an error.

Without proper CI on Apple platforms (🥲), I don't know how to shake those out, so I guess we'll have to land this and be ready to fix stuff forward, and potentially revert.

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