swift234

Runtime: Tweak Mirrors for resilience

Make _MagicMirrorData @_fixed_layout, but not the concrete mirror
implementations.

To make the calling convention work in resilient builds, make the
runtime entry points into top-level functions that take a
_MagicMirrorData, instead of adding @_silgen_name attributes on
methods.

This involves changing the convention on the 'owner' parameter
from +0 to +1.

Also, there was a memory leak in the old enum code that I noticed
by inspection. We would copy the enum value into a box, strip
the enum tag bits, take the box contents but never free the box
itself.

The fix isn't very satisfying either -- we have to modify the
source enum in-place to strip tag bits, then we copy it into a
box having the right payload type, and add the tag bits back.
At least this way, we can free the box after.