add Array.contains and refactor checkClosure in semantic3.d

[gorsing]

Summary

This PR adds the contains predicate to Array in dmd.root.array and applies it to refactor semantic analysis logic.

In the compiler's semantic stages, we often use manual foreach loops combined with label-breaks to avoid duplicates. By introducing contains, we simplify this pattern, reducing cognitive load and making the programmer's intent explicit without introducing side-effect obfuscation.

Key Changes

• dmd.root.array: Added contains method (and underlying find logic) to the Array struct.
• dmd.semantic3: Refactored checkClosure to use a.contains(f). This replaces a manual loop and a labeled break, significantly cleaning up the logic for tracking captured nested functions.

[dlang-bot]

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#22300"