Rollup merge of #61615 - petrochenkov:errlit, r=matklad

syntax: Treat error literals in more principled way

Free them from their character literal origins.

I actually tried to remove `LitKind::Err` entirely (by converting it into `ExprKind::Err` immediately), and it caused no diagnostic regressions in the test suite.
However, I'd still want to use error literals as general purpose error tokens some day, so I kept them.

The downside of having `LitKind::Err` in addition to `ExprKind::Err` is that every time you want to do something with `ExprKind::Err` you need to make sure that `ExprKind::Lit(LitKind::Err)` is treated in the same way.
Fortunately, this usually happens automatically because both literals and errors are "leaf" expressions, however this PR does fix a couple of inconsistencies between them.

Addresses https://github.com/rust-lang/rust/pull/60679#discussion_r282640663 in a way

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