compiler-blacklist-versions: simplify clang blacklisting by associating Apple clang versions with macports-clang versions

[kencu (Ken)]

Long title, apologies.

I would be helpful if we could find an association between Apple clang releases and macports-clang-N versions such that blacklisting a given version of clang, such as {clang < 800} would automatically also blacklist macports-clang-3.8 and below (or whichever version best correlates).