Restore r184205 and associated commits (after commit of r185290)

This allows clang to use the backend parameter attribute 'returned' when generating 'this'-returning constructors and destructors in ARM and MSVC C++ ABIs.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185291 91177308-0d34-0410-b5e6-96231b3b80d8
Documentation cleanup for TypeOrdering.h.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185293 91177308-0d34-0410-b5e6-96231b3b80d8