[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 not link time that's the problem but load/run time due to the `@loader_path` in its `LC_RPATH`.
> 
> I don't understand. There's no RPATH added by this patch.

Sorry it's not the the `@loader_path` in the `LC_RPATH` in the Xcode libraries that's the issue, it's that their `LC_ID_DYLIB` are all `@rpath` based, e.g. `@rpath/libclang.dylib`. That basically means they aren't usable outside of Xcode because you wouldn't be able to resolve `@rpath` to a given Xcode installation.

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