Auto merge of #51727 - varkor:expragain-to-exprcontinue, r=petrochenkov

Auto merge of #51727 - varkor:expragain-to-exprcontinue, r=petrochenkov

Rename hir::ExprAgain to hir::ExprContinue

The current name is confusing and historical.

I also used this PR to clean up the annoying indentation in `check/mod.rs`. If that's viewed as too tangential a change, I'll split it up, but it seemed reasonable to slip it in to reduce @bors's work. It's easy to compare for the two commits individually.

r? @petrochenkov