[SR-4330] Use reference-counted copy-on-write existential buffers

[bob-wilson]

Previous ID	SR-4330
Radar	rdar://problem/22591042
Original Reporter	@bob-wilson
Type	Improvement
Status	Resolved
Resolution	Done

Additional Detail from JIRA

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

md5: f4fe9f3489eca1390e7a47d05b48b7a5

Issue Description:

This tracks work to use reference-counted copy-on-write buffers when the payload of an existential does not fit in the inline buffer. This will avoid some malloc() allocation of buffers in exchange for reference counts and uniqueness checks, which we expect to be an overall win for performance.

[milseman]

aschwaighofer@apple.com (JIRA User), this was done, correct?

[aschwaighofer]

This was done for swift 4.