Abstract out parts of thunk emission code, add support for simple thunks when using -cxx-abi microsoft

Reviewed at http://llvm-reviews.chandlerc.com/D1787

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@192220 91177308-0d34-0410-b5e6-96231b3b80d8