[libcxx] [libunwind] [llvm] [libc++] Update macOS and AppleClang (PR #193711)

[Anthony Latsis via llvm-commits]

AnthonyLatsis wrote:

The high bar here tends to cause us problems when we start adopting a soon-to-be-cut llvm-project release branch because Swift CI builders seldom run the latest clang release. Upgrading these builders and fixing the fallout takes time, while the 26.4 release and the rebranching start date are just a few weeks apart.

https://github.com/llvm/llvm-project/pull/193711