Revert "llvm-3.9: Obsolete port" and "llvm-4.0: Obsolete port"
This reverts commits 8470ccfcdae5c31638c4c5bc2b24ee05e4a2a95b and
7615202b212d36d0598b5775cb33020a916462ac.
These clang versions are in the default fallback list used by base,
and so need to remain available until after a new base version that
doesn't use them is released.
Fixes: https://trac.macports.org/ticket/58741
