Make thunk this/return adjustment ABI-specific. Also, fix the return adjustment when using -cxx-abi microsoft

Reviewed at http://llvm-reviews.chandlerc.com/D2026

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@193679 91177308-0d34-0410-b5e6-96231b3b80d8