choose a less arbitrary span when parsing the empty visibility modifier

Visibility spans were added to the AST in #47799 (d6bdf296) as a
`Spanned<_>`—which means that we need to choose a span even in the case
of inherited visibility (what you get when there's no `pub` &c. keyword
at all). That initial implementation's choice is pretty
counterintuitive, which could matter if we want to use it as a site to
suggest inserting a visibility modifier, &c.

(The phrase "Schelling span" in the comment is meant in analogy to the
game-theoretic concept of a "Schelling point", a value that is chosen
simply because it's what one can expect to agree upon with other agents
in the absence of explicit coördination.)

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