Implement Vec::from_fn

[EFanZh]

• ACP: Support vec![const { ... }; n] syntax for creating a Vec of non-Clone values libs-team#484 (comment).
• Tracking issue: Tracking Issue for Vec::from_fn #149698.

[rustbot]

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[rustbot]

This PR was rebased onto a different main 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.

[joboet]

@bors r+

[bors]

📌 Commit c5113ca has been approved by joboet

It is now in the queue for this repository.