Issue Description:

The PR for https://bugs.swift.org/browse/SR-170 implements a half baked multiline string literal.

There are two issues remaining, however this issue report is only about the first one:

1. Warning about trailing whitespaces.

2. Trailing backslash to escape new lines:

• https://github.com/hartbit/swift-evolution/blob/literal-string-improvements/proposals/XXXX-improve-string-literals.md

• Newline escapes in Swift Strings swift-evolution#695

However Joe Groff said in the following posts, that it's a reasonable thing to warn about:

• https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170417/035931.html

• https://lists.swift.org/pipermail/swift-evolution/Week-of-Mon-20170417/035934.html

The following code sample is misleading and has to emit a warning, either to remove the trailing whitespaces manually or providing a `Fix-it` that will trim them.

let s = """
    abc                                                               
    """

print(s.characters.count) // prints 66

The current workaround without the trailing backslash if the white spaces are desired could look like this:

let s = """
    abc                                                               \("")
    """

print(s.characters.count) // prints 66