[SR-7016] Swift should set dso_local for symbols

[bob-wilson]

Previous ID	SR-7016
Radar	rdar://problem/37627393
Original Reporter	@bob-wilson
Type	Task

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Task
Assignee	None
Priority	Medium

md5: e8edc8b784810fb4b7c82993eab77ab7

Issue Description:

This is currently only relevant to the master-next branch of Swift, which works with the latest version of LLVM. Recent work on LLVM adds support distinguishing runtime-preemptable symbols by marking them as either dso_local or dso_preemptable. Setting those flags correctly probably requires some work in the Swift frontend. Compare with Clang r324535.

[bob-wilson]

I made some related changes in #14572 but that was just enough to get one test to pass.

[bob-wilson]

@swift-ci create