in which unused-parens suggestions heed what the user actually wrote

in which unused-parens suggestions heed what the user actually wrote

Aaron Hill pointed out that unnecessary parens around a macro call
(paradigmatically, `format!`) yielded a suggestion of hideous
macro-expanded code. (The slightly unusual choice of using the
pretty-printer to compose suggestions was quite recently commented on
in the commit message for 1081bbbfc ("abolish ICE when pretty-printing
async block"), but without any grounds to condemn it as a 𝘣𝘢𝘥
choice. Hill's report provides the grounds.) `span_to_snippet` is
fallable as far as the type system is concerned (because, who knows,
macros or something), so the pretty-printing can live on in the
oft-neglected `else` branch.

Resolves #55109.