Fix rewrite of multiline string literals.

This commits ensures that we don't rewrite `\'-terminated
multiline string literals. Also, it fixes the processing
of \r characters inside the string literals.

Change-Id: If3d7c1b83c7306b9ccb1be31412b6f8e76434c41
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@nokia.com>
Reviewed-by: Aaron Kennedy <aaron.kennedy@nokia.com>