Update Clang on MacOS Darwin

This is what it needs to update Clang on older MacOS (Clang-17 on Mojave in this example):

• Install MacPorts package from https://www.macports.org/install.php
• Install new Clang version:

sudo port install clang-17

• Install SDK headers (https://stackoverflow.com/a/55465527/23185349):

open /Library/Developer/CommandLineTools/Packages/macOS_SDK_headers_for_macOS_10.14.pkg

• Add compiler's binary to PATH:

export PATH=/opt/local/libex/llvm-17/bin/:$PATH