[SR-2582] Add -swift-version command line flag

[jtbandes]

Previous ID	SR-2582
Radar	rdar://problem/29054415
Original Reporter	@jtbandes
Type	New Feature
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	New Feature
Assignee	graydon (JIRA)
Priority	Medium

md5: 69481d2da35f1f5645d3bfb5ecf7eb8c

Issue Description:

Per the plan for Swift 3, 4, and beyond, the compiler will need to support older versions of the Swift language.

The best spelling anyone has come up with so far is a -swift-version 3 flag. This should be parsed and piped into LangOptions.

https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160905/026944.html
https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20160725/025699.html

[belkadan]

@rjmccall, are you still going to be working on this?

[rjmccall]

No, someone else should probably take it.

[swift-ci]

Comment by Graydon Hoare (JIRA)

Looking at it now.

[swift-ci]

Comment by Graydon Hoare (JIRA)

PR #4826
Merged ea5aaa6