The purpose of the newline is to make it clearer where a given
diagnostic begins and ends, particularly as the first message of a
diagnostic may not be the error.
This is a trivial code change, but ripples edits through test files.
Note #3486 rewrites the macro behavior, and is already approved: so this
PR is only for the changed enforcement during error. Also, #3493 already
changed several things to allow any token while this PR was awaiting
review, but this still changes enforcement for `For` and `If`.
This was brought up on
[#toolchain](https://discord.com/channels/655572317891461132/655578254970716160/1182066616456970251),
and I think this any-on-error approach gets at least some support. We
could try setting it to the introducer, but it's quite possible we want
it to be something like the token which led to the parse error, rather
than a static token. That leads to a conclusion that, most typically,
we'll expect arbitrary tokens when error conditions may lead to tokens
which aren't the expected token.
A couple related, recent `CARBON_IF_ERROR` crash fixes can be found in
#3404 and #3424. Something like #3404 would've been needed regardless
because `namespace` didn't have `CARBON_IF_ERROR` before, although I
might've missed the underlying issue with declarations because only
`namespace` had a relevant test (that is, if #3404 had added
`CARBON_ANY_TOKEN_ON_ERROR`, I wouldn't have had a crash in #3462).
#3424 would've been avoided with this change because there was a
`CARBON_IF_ERROR`, and it was just too restrictive.
Jon Ross-Perkins