[SR-9311] SourceKit-LSP should not use String.Index.encodedOffset as UTF16 offset

[rintaro]

Previous ID	SR-9311
Radar	None
Original Reporter	@rintaro
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	SourceKit-LSP
Labels	Bug
Assignee	None
Priority	Medium

md5: 8dfc9af638c0545d5e232c1909733399

Issue Description:

String.Index.encodedOffset doesn't necessarily mean UTF16 offset.

In Swift4.2 on macOS, '"こんにちは".endIndex.encodedOffset' results 5, but in the current master, it's 15 probably because swiftlang/swift#20315 .

Moreover, if the string is backed by NSString it's 5 even in Swift5.

let str: String = "こんにちは"
print(str.endIndex.encodedOffset) // 15

import Foundation
let nsStr: NSString = "こんにちは"
let _nsStr = nsStr as String
print(_nsStr.endIndex.encodedOffset) // 5

[rintaro]

CC: @benlangmuir

[rintaro]

#14