We now use invisible delimiters for expanded `vis` fragments, instead of
`Token::Interpolated`.

Notes about tests:

- tests/ui/parser/macro/trait-object-macro-matcher.rs: the syntax error
  is duplicated, because it occurs now when parsing the decl macro
  input, and also when parsing the expanded decl macro. But this won't
  show up for normal users due to error de-duplication.

- tests/ui/associated-consts/issue-93835.rs: ditto.

- The changes to metavariable descriptions in this PR's earlier commits
  are now visible in error message for several tests.

The one notable test change is `tests/ui/macros/trace_faulty_macros.rs`.
This commit removes the complicated `Interpolated` handling in
`expected_expression_found` that results in a longer error message. But
I think the new, shorter message is actually an improvement.

The original complaint was in rust-lang#71039, when the error message started
with "error: expected expression, found `1 + 1`". That was confusing
because `1 + 1` is an expression. Other than that, the reporter said
"the whole error message is not too bad if you ignore the first line".

Subsequently, extra complexity and wording was added to the error
message. But I don't think the extra wording actually helps all that
much. In particular, it still says of the `1+1` that "this is expected
to be expression". This repeats the problem from the original complaint!

This commit removes the extra complexity, reverting to a simpler error
message. This is primarily because the traversal is a pain without
`Interpolated` tokens. Nonetheless, I think the error message is
*improved*. It now starts with "expected expression, found `pat`
metavariable", which is much clearer and the real problem. It also
doesn't say anything specific about `1+1`, which is good, because the
`1+1` isn't really relevant to the error -- it's the `$e:pat` that's
important.

This involves replacing `nt_pretty_printing_compatibility_hack` with
`stream_pretty_printing_compatibility_hack`.

The handling of statements in `transcribe` is slightly different to
other nonterminal kinds, due to the lack of `from_ast` implementation
for empty statements.

Notable test changes:
- `tests/ui/proc-macro/expand-to-derive.rs`: the diff looks large but
  the only difference is the insertion of a single invisible-delimited
  group around a metavar.

This time when converting them to proc-macro `Group` form.

Note: there was an existing code path involving `Interpolated` in
`MetaItem::from_tokens` that was dead. This commit transfers that to the
new form, but puts an `unreachable!` call inside it.

Notes about tests:
- tests/ui/rfcs/rfc-2294-if-let-guard/feature-gate.rs: some messages are
  now duplicated due to repeated parsing.

- tests/ui/rfcs/rfc-2497-if-let-chains/disallowed-positions.rs: ditto.

- `tests/ui/proc-macro/macro-rules-derive-cfg.rs`: the diff looks large
  but the only difference is the insertion of a single
  invisible-delimited group around a metavar.

`NtBlock` is the last remaining variant of `Nonterminal`, so once it is
gone then `Nonterminal` can be removed as well.

They are no longer needed.

This does slightly worsen the error message for a single test, but that
test contains code that is so badly broken that I'm not worried about
it.