Remove unnecessary environment switch

All supported environments on x86 Windows return structs in EAX:EDX.
This removes code added in r204978 that had to get updated in r222680.
We should now have the same behavior we had before r204978.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@222697 91177308-0d34-0410-b5e6-96231b3b80d8