Merge changes to clang's Driver code into LLVM's Option library

This is in preparation for switching the clang driver over to using LLVM's
Option library.  Richard Smith introduced most of these changes to the clang
driver in r167638.

Reviewers: espindola on IRC

Differential Revision: http://llvm-reviews.chandlerc.com/D970

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183925 91177308-0d34-0410-b5e6-96231b3b80d8