[Option] Add inclusion and exclusion flags to option parsing

Summary:
This allows the clang driver to put MSVC compatible options in the same
enumerator space as its normal options but exclude them from normal
option parsing.

Also changes the standard ParseArgs() method to consider unknown
arguments with a leading slash as being inputs rather than flags.

High level discussion for clang-cl is here:
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2013-June/030404.html

CC: llvm-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D1049
Add some flag exclusion tests.