MS ABI: Use the correct this arg when generating implict copy ctor

We assumed that the incoming this argument would be the last argument.

However, this is not true under the MS ABI.

This fixes PR20897.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@217642 91177308-0d34-0410-b5e6-96231b3b80d8