Creates a special-comment value that encapsulates v. The
read, read-syntax, etc., procedures treat values
constructed with make-special-comment as delimiting
whitespace when returned by a reader-extension procedure (see
Reader-Extension Procedures).
Returns the value encapsulated by the special-comment value
sc. This value is never used directly by a reader, but it
might be used by the context of a read-char-or-special, etc.,
call that detects a special comment.