Print `token::Interpolated` with token stream pretty printing.

Instead of using AST pretty printing.

This is a step towards removing `token::Interpolated`, which will
eventually (in #124141) be replaced with a token stream within invisible
delimiters.

This changes (improves) the output of the `stringify!` macro in some
cases. This is allowed. As the `stringify!` docs say: "Note that the
expanded results of the input tokens may change in the future. You
should be careful if you rely on the output."

Test changes:

- tests/ui/macros/stringify.rs: this used to test both token stream
  pretty printing and AST pretty printing via different ways of invoking
  of `stringify!` (i.e. `$expr` vs `$tt`). But those two different
  invocations now give the same result, which is a nice consistency
  improvement. This removes the need for all the `c2*` macros. The AST
  pretty printer now has more thorough testing thanks to #125236.

- tests/ui/proc-macro/*: minor improvements where small differences
  between `INPUT (DISPLAY)` output and `DEEP-RE-COLLECTED (DISPLAY)`
  output disappear.

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