Rollup merge of #81869 - mark-i-m:leading-vert, r=petrochenkov

Rollup merge of #81869 - mark-i-m:leading-vert, r=petrochenkov

Simplify pattern grammar, improve or-pattern diagnostics

This implements the change under FCP in https://github.com/rust-lang/rust/issues/81415. It allows nested or-patterns to contain a leading `|`, simplifying the [grammar for patterns](https://github.com/rust-lang/reference/pull/957/files?short_path=cc629f1#diff-cc629f15712821139bc706c63b3845ab59a008e2a998e08ffad42e3aebcbcbe2).

Along the way, we also improve the diagnostics around a few specially-handled cases, such as using `||` instead of `|`, using or-patterns in fn params, including the leading `|` in the pattern span, etc.

r? `@petrochenkov`