Revert "Move using directive from Job.h to Job.cpp" (r190902)

Revert "Move using directive from Job.h to Job.cpp" (r190902)

Seems like it was intentional to export ArgStringList as
driver::ArgStringList, and e.g. examples/clang-interpreter/main.cpp
uses it this way.

However, exporting it with a typedef seems like a more common way to do it.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@190906 91177308-0d34-0410-b5e6-96231b3b80d8