[Migrated] Allow keeping linkage annotation

Issue automatically imported from old repo: https://github.com/EmbarkStudios/rust-gpu/issues/1042
Old labels: t: enhancement
Originally creatd by SiebenCorgie on 2023-04-21T16:42:49Z

SpirV allows defining import and export [0] for linking separate Shader modules [1] post compilation.

Apparently SpirV linking is already used within rust-gpu, but not exposed. Therefore we can't use extern "C" as we usually would.

This functionality would be useful, for instance when building shader permutation systems.

The enhancement would basically be allowing extern C, #[no_mangle] etc. to map to (possibly named) import/export decorated values whenever the Linkage capability on spirv-builder is enabled.

Inizia dalla capability Linkage di spirv-builder ed esamina la specifica di linkage SPIR-V collegata. Traccia il modo in cui rust-gpu gestisce attualmente i valori extern "C" e #[no_mangle", quindi determina la mappatura necessaria alle decorazioni di import/export denominate. Il lavoro è completato quando le annotazioni di linkage vengono preservate con Linkage abilitato e moduli shader separati possono utilizzarle.