linker/duplicates: handle all decorations instead of special-casing zombies.

[eddyb]

While trying to avoid deduplication in some cases (where it destroys useful information for debugging "what specific part of the source code caused this invalid type to be generated"), I came across the special-casing for zombies in make_dedupe_key (which avoids other unwanted cases of zombie deduplications).

But gather_annotations should be processing all decorations including the OpDecorateString used for the "custom decorations" system that includes zombies, it just looks like an accidental omission in the past.

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The other change here is to ignore some "type mismatches" that result from slightly-different zombies annotations showing up on otherwise-identical types from different crates (likely the first Span passed to SpirvType::def gets cached, so the codegen order influences this).

The reason they're safe to ignore is that "zombies" (i.e. deferred errors) have only two possible outcomes:

• if sufficiently "kept around" (i.e. "not dead"): the error is actually emitted
  • (and is likely more useful than any "meta errors" about some kind of mismatch in the details of that error)
• if not actually needed (i.e. "dead code"): it's removed from the SPIR-V module entirely
  • (and so any type issues within that definition go away with the definition itself)