GNUstep Objective-C ABI version 2
This includes initial support for the (hopefully final) updated Objective-C ABI, developed here:
It also includes some cleanups and refactoring from older GNU ABIs.
The current version is ELF only, other formats to follow.
Reviewers: rjmccall, DHowett-MSFT
Reviewed By: rjmccall
Subscribers: smeenai, cfe-commits
Differential Revision: https://reviews.llvm.org/D46052