MS ABI: Return sret parameters when using inalloca

Previously the X86 backend would look for the sret attribute and handle
this for us.  inalloca takes that all away, so we have to do the return
ourselves now.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@202097 91177308-0d34-0410-b5e6-96231b3b80d8