NonAsciiCharactersAreEscapedInLiterals issue with Verbatim Stings

[ellismg]

The current rule will convert @"This is a snowman: ☃." to: @"This is a snowman: \u2603.". This is wrong because we want the unicode literal in the string, not the escaped version. Instead it should convert it to something like:

@"This is a snowman: " + "\u2603" + @"."

Note that to be nice we should probably compress runs of Unicode characters together if they appear one after another.