It sounds like you have the cctools port (the one that provides as) installed with one of the +llvm variants (which used to be the default on all systems). You can verify whether that is so by looking at the output of:
port installed cctools
You didn't mention what version of Xcode or macOS you have, but if you're using Xcode 9 or later, please reinstall the cctools port with the +xcode variant instead, which is as of 12 days ago the new default when Xcode 9 or later is used (but unfortunately there isn't a mechanism in MacPorts that allows us to make that new default available to any users who had already installed the port before). To do that, run:
sudo port selfupdate
sudo port install cctools +xcode
