GNUstep Objective-C ABI version 2

Summary:
This includes initial support for the (hopefully final) updated Objective-C ABI, developed here:

https://github.com/davidchisnall/clang-gnustep-abi-2

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

llvm-svn: 332950