Only allow Checked C extension flag for C in clang #9
Description
This issue was copied from checkedc/checkedc-clang#9
We should only allow the Checked C extension flag to be used for C in clang. We should issue an error message for other languages. We're not currently doing the implementation work to enable all the language features for other C family languages supported by clang (such as C++). We're also not testing the other C family languages.