MS ABI: Fix __fastcall methods that return structs

The sret paramater consumes the register after the implicit 'this'
parameter, as with other calling conventions.

Fixes PR20278, which turned out to be very easy.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@212669 91177308-0d34-0410-b5e6-96231b3b80d8