[ABISysV] Fix regression for Simulator and MacABI

The ABISysV ABI was refactored in r364216 to support the Windows ABI for
x86_64. In particular it changed ABISysV_x86_64::CreateInstance to
switch on the OS type. This breaks debugging MacABI apps as well as apps
in the simulator. This adds back the necessary cases.

We have a test on Github that exercises this code path and which I'd
like to upstream once the remaining MacABI parts become available in
clang.

Differential revision: https://reviews.llvm.org/D67869

llvm-svn: 372642