Rename `/P` to `/Po` and match `cl.exe` `/P` behavior

[llvm-beanz]

DXC's /P flag doesn't match the behavior of cl.exe, which many other DXC flags do. Perhaps we can rename /P to /Po to preserve the existing behavior and make /P behave as cl.exe.

[damyanp]

This is worth doing to further align dxc with clang-dxc.