Option parsing: recognize the special -- token

Everything that comes after -- should be treated as a filename. This
enables passing in filenames that would otherwise be conflated with
command-line options.

This is especially important for clang-cl which supports options
starting with /, which are easily conflatable with Unix-style
path names.

Differential Revision: http://llvm-reviews.chandlerc.com/D1274

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@187675 91177308-0d34-0410-b5e6-96231b3b80d8