Refactor UDA Handling: Improve Safety and Support Single Attributes

[gorsing]

Description: This PR refactors the User Defined Attribute (UDA) semantic analysis logic within dmd.attribsem. The primary goals are to improve compiler stability and ensure consistent attribute iteration.

Changes:

getAttributes:

Added early exit to avoid heap allocation for empty attribute blocks.
Improved recursive collection of attributes from parent declarations.

foreachUda:

Bug Fix: Added null-check for the attribute list to prevent crashes during semantic analysis.
Feature: Implemented support for single (non-tuple) attributes. Previously, standalone UDAs might have been skipped during iteration.

Cleanup: Removed redundant/unreachable code blocks and unified the iteration logic using each!.

[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#22287"

[thewilsonator]

If this fixes bugs, please add their reproductions to the test suite

[gorsing]

@thewilsonator Thanks for the feedback! I've updated the PR to address all your comments:

Code Cleanup: Removed all redundant comments (// Optimization, // PROTECTION, etc.) as they were unnecessary.

Ddoc Formatting: Restored and fixed the indentation in the documentation blocks.

Logic Refinement: Restructured foreachUda to use an explicit else branch for handling single UDA expressions, making the flow clearer.

Regression Test: Added a new test case compiler/test/runnable/test22287.d which covers single UDAs, tuple UDAs, and recursive attribute collection.