[clang-format] Handle NSString literals by merging tokens.

Summary:
This fixes a few outstanding bugs:
  * incorrect breaking of NSString literals containing double-width characters;
  * inconsistent formatting of ObjC dictionary literals containing NSString
    literals;
  * AlwaysBreakBeforeMultilineStrings ignoring implicitly-concatenated NSString
    literals.

Reviewers: djasper

Reviewed By: djasper

Subscribers: klimek, cfe-commits

Differential Revision: https://reviews.llvm.org/D31706

llvm-svn: 299927