X86: balance the frame prologue and epilogue on Win64

This was broken in ba1509da.  The Win64
frame would not perform the setup of the Swift async context parameter
but would tear down the setup in the epilogue resulting in crashes.
This ensures that we do the full setup when we do the tear down.
Although this is non-conforming to the Win64 calling convention, it
corrects the setup and exposes the actual issue that the change
introduced: incorrect frame setup.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D104246