Compilation: Clean up addCCArgs().

The goal of this commit is to get rid of some "unused command line argument"
warnings that Clang would give for various file types previously. This cleanup
also has the side effect of making the order of flags more understandable,
especially as it pertains to include paths.

Since a lot of code was shuffled around in this commit, I recommend reviewing
the old and new versions of the function side-by-side rather than trying to make
sense of the diff.