swift234

Only use metadata patterns for generic types; perform other
initialization in-place on demand.  Initialize parent metadata
references correctly on struct and enum metadata.

Also includes several minor improvements related to relative
pointers that I was using before deciding to simply switch the
parent reference to an absolute reference to get better access
patterns.

Includes a fix since the earlier commit to make enum metadata
writable if they have an unfilled payload size.  This didn't show
up on Darwin because "constant" is currently unenforced there in
global data containing relocations.

This patch requires an associated LLDB change which is being
submitted in parallel.