MC: Better management of macro arguments

The linux kernel makes uses of a GAS `feature' which substitutes nothing
for macro arguments which aren't specified.

Proper support for these kind of macro arguments necessitated a cleanup of
differences between `GAS' and `Darwin' dialect macro processing.

Differential Revision: http://llvm-reviews.chandlerc.com/D2634


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@200409 91177308-0d34-0410-b5e6-96231b3b80d8