FIX #56335 [Augmentations] Analyzer doesn't support augmenting functions with empty bodies

[Darkildo]

This PR fixes analyzer handling for augmenting functions with empty bodies.

According to the augmentation specification, an augmenting function declaration may use an empty body (;) to augment only metadata or documentation comments, without changing the body of the augmented member. In analyzer, this currently led to a false positive on code like:

class C {
  void foo() {}
}

augment class C {
  augment void foo();
}

The root cause was that the analyzer scanner only enabled augmentation-library scanning when Feature.macros was enabled. That caused augmentation syntax to be scanned incorrectly unless macros were enabled, which then cascaded into bogus parse/analysis errors, including CONCRETE_CLASS_WITH_ABSTRACT_MEMBER.

This change updates the scanner to use Feature.augmentations for forAugmentationLibrary, which matches the feature actually being tested, and adds a regression test to verify that an empty-body augmenting method no longer reports an error.

Fix #56335

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