[SR-3924] Review/cleanup the type metadata representation

[ematejska]

Previous ID	SR-3924
Radar	rdar://problem/31414916
Original Reporter	@ematejska
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AffectsABI
Assignee	None
Priority	Medium

md5: 8841e5387521c7e1897123d5742ed327

Issue Description:

Type metadata has many historical artifacts in its representation. Review and clean these up ahead of declaring ABI Stability.

[bob-wilson]

@swift-ci create

[bob-wilson]

From Joe Groff: "All the big layout changes we had planned should be done at this point."