Implement base class upcast Debug formatting.

[copybara-service]

Implement base class upcast Debug formatting.

This CL adds support for formatting C++ base classes in generated Rust Debug
implementations (under the experimental feature flag).

Key behaviors:

• When generating Debug for a C++ record, if the target has experimental enabled, it formats all public, unambiguous, non-virtual base classes that themselves implement Debug.
• Bases are formatted as anonymous components using an empty string for the field name (e.g., derived_field("", &self.base_field)), which formats as : Base in Rust (e.g. PubliclyDerived { : Base, .. }).
• Refactors cc_struct_upcast_impl to return the list of successfully verified upcastable base records, allowing the formatting generator to reuse the exact same validation and upcasting logic.
• Isolates experimental inheritance tests into a separate derived_debuggables library and derived_debuggables_test target in test/debug/, enabled only with the experimental aspect hint.
• Regenerates inheritance golden files to verify the anonymous base formatting.
• Adds a unit test test_template_specialization_override_debug_propagation in ir_from_cc_test.rs to verify that the crubit_override_debug annotation propagates correctly to template specializations used as bases (working around the importer's template traversal limitation using C++ type aliases).

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