On invalid parse nodes, if the token may vary, allow any token.

[jonmeow]

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, 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.

[jonmeow]

(note, dependencies in stack)

[jonmeow]

Note, some changes here overlapped with #3493, which would've incidentally fixed the namespace crash. However, since #3493 still didn't change everything do CARBON_IF_ERROR(CARBON_ANY_TOKEN), this is still a question.

[zygoloid]

I think this makes sense. The prior behaviour doesn't seem especially useful to me, given that if you wanted the for or if token you can easily find them.