[MC] [Win64EH] Try to generate packed unwind info where possible

In practice, this only gives modest savings (for a 6.5 MB DLL with
230 KB xdata, the xdata sections shrinks by around 2.5 KB); to
gain more, the frame lowering would need to be tweaked to more often
generate frame layouts that match the canonical layouts that can
be written in packed form.

Differential Revision: https://reviews.llvm.org/D87371