Driver: Use the canonical command line arguments.

Summary:
Use the arguments given to the OS at process creation-time instead of
the arguments passed into main() by the C runtime environment.  The ones
that main() received may not be suitable (e.g. not Unicode).

Depends on D1834

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1835

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@192091 91177308-0d34-0410-b5e6-96231b3b80d8