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

We assumed the last argument of the copy constructor was the this
pointer.  However, this is not the case under the MS ABI.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@219775 91177308-0d34-0410-b5e6-96231b3b80d8