I have tried to upgrade my macports after upgrading my iMac to Sierra. When Clang-3.3 was encountered, port upgrade gave the message: llvm-3.3 is not supported on macOS Sierra or newer and stopped further upgrade process.
Had Macports stopped producing Clang-3.3 (and its dependents) for Sierra?
From this ticket, and the Portfiles of llvm-3.6 and earlier, I would conclude that you'll need at least llvm-3.7
if you want it to install on Sierra.