A vl-propexpr that is just true.
This is useful as the implicit first step in a vl-propthen that a user might write without a leading expression. That is, if the user writes something like:
##1 foo ##1 bar
Then this is equivalent to having written:
1 ##1 foo ##1 bar
To simplify our internal representation, we always make this leading
Similarly, our internal form of
Definition:
(defconst *vl-trivially-true-property-expr* (make-vl-propcore :guts (make-vl-exprdist :expr (vl-make-index 1))))