Add comment about exporting clang::driver::ArgStringList,
as suggested by Jordan on IRC. Also, use the unqualified name
in Job.cpp.

And while we're here, refer to StringRef with the unqualified
name, because we have a using directive for that too.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190909 91177308-0d34-0410-b5e6-96231b3b80d8