[Driver] Refactor clang driver to use LLVM's Option library

The big changes are:
- Deleting Driver/(Arg|Opt)*
- Rewriting includes to llvm/Option/ and re-sorting
- 'using namespace llvm::opt' in clang::driver
- Fixing the autoconf build by adding option everywhere

As discussed in the review, this change includes using directives in
header files.  I'll make follow up changes to remove those in favor of
name specifiers.

Reviewers: espindola

Differential Revision: http://llvm-reviews.chandlerc.com/D975