[SR-4353] Use word-size field in native object header for reference counting

[bob-wilson]

Previous ID	SR-4353
Radar	rdar://problem/29765800
Original Reporter	@bob-wilson
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Improvement, AffectsABI
Assignee	@gparker42
Priority	Medium

md5: acede700f07b312a6e3ed4b7bd66e0e3

Issue Description:

The plan for the stable ABI is to have a word-sized field in the native object header that can be used for reference counting. So far, we have had a fixed-size 64-bit field for that purpose. Greg Parker's work on the new reference counting implementation (#5282 sets the stage for this, but we still need to shrink the size of that field on 32-bit targets.

[gparker42]

WIP: #7837

Need to correct one place in the reflection (`readMetadataSource` in ReflectionContext.h) and coordinate with the debugger and tools.

[bob-wilson]

Greg did this in #12790