[clang] [clang] Match -isysroot behaviour with system compiler on Darwin (PR #80524)

[Louis Dionne via cfe-commits]

ldionne wrote:

In https://reviews.llvm.org/D157283#4648242, I explained that the current upstream code actually matches exactly what we have downstream. I made sure to upstream everything to avoid differences after we started shipping libc++ in the SDK and were done with our transition.

@drodriguez  I'm not certain why you think AppleClang prefers `isysroot` over toolchain headers -- it might be because we simply don't ship libc++ toolchain headers anymore and we only ship libc++ SDK headers. But if you try installing libc++ headers in the toolchain alongside a reasonably recent AppleClang, it should prefer them over the SDK headers (which is consistent with the upstream behavior).

https://github.com/llvm/llvm-project/pull/80524