Proposal for multi-line string literal syntax.

[johnno1962]

Originally discussed in pull request #322 and on Swift evolution this PR puts forward a single proposal as suggested by coments on the previous PR for multi-line string literals now it is in scope for Swift 4 stage 2. The implementation has been explored in a previous pull request swiftlang/swift#2275 and is a relatively minor change if agreement on the approach to take could be reached.

[DevAndArtist]

+1 for Continuation quotes, because it's more precise and solves issues with the indention for both the literal and its multiline content.

I also wanted to add a note that one could possibly use use strings in general like this:

let multilineString: String = "123""__456__" // Multiline literal in a single line (fixit?)
     "__789__" // replace any underscore with space characters
     "_____0_" // precise control about pre- and post-space-characters

// Multiline literal in a single line allows comments in between
let otherString = "\(someInstance)" /* comment */ "text \(someOtherInstance) text"

[DevAndArtist]

Or using the escaping character, similar to Javascript:

let multilineString: String = "123__456__\ // indicates there is another part of the string on the next line
                              "__789_____\ // aways starts with `"` and ends with either `\` or `"`
                              "_____0_" // precise control about pre- and post-space-characters

let otherString = "\(someInstance)\ /* only comments are allowed in between */ "text \(someOtherInstance) text"