Introduce helper functions for Some(x)/None patterns and expressions

[ada4a]

Reduces boilerplate a bit

changelog: none

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[ada4a]

This needs to depend on #15929 for a somewhat annoying reason: the spans in unnecessary_filter_map are completely borked, and this PR happens to change the span from one broken version to another, which ui_test complains about. Oh well.

@rustbot blocked

[rustbot]

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.