read syntax for multibyte strings

[Dave Love]

I wanted to do this to produce a multibyte string constant:

      (while (and (skip-chars-forward
                   (eval-when-compile   ; missing optimization
                     (string-as-multibyte "^\341-\377"))
                   range)
                  (not (eobp)))

It works if you do `compile-defun', but not from a byte-compiled file.
The reason is that there's no way to specify the multibyte form of the
string constant read from the file.  It might be useful to have read
syntax to do so in cases like this.