When diagnosing a duplicated name, add the name to the diagnosis (#4902)

In order to have the name available for diagnostics, we now always set
`NameId` in `NameContext` and put `poisoning_loc_id` as part of the
union with `resolved_inst_id` instead (since we never need both).

toolchain/check/import_ref.cpp

@@ -138,7 +138,8 @@ auto GetCanonicalImportIRInst(Context& context, SemIR::InstId inst_id)
   return GetCanonicalImportIRInst(context, &context.sem_ir(), inst_id);
 }
 
-auto VerifySameCanonicalImportIRInst(Context& context, SemIR::InstId prev_id,
+auto VerifySameCanonicalImportIRInst(Context& context, SemIR::NameId name_id,
+                                     SemIR::InstId prev_id,
                                      SemIR::ImportIRInst prev_import_ir_inst,
                                      SemIR::ImportIRId new_ir_id,
                                      const SemIR::File* new_import_ir,
@@ -151,7 +152,7 @@ auto VerifySameCanonicalImportIRInst(Context& context, SemIR::InstId prev_id,
   auto conflict_id =
       AddImportRef(context, {.ir_id = new_ir_id, .inst_id = new_inst_id});
   // TODO: Pass the imported name location instead of the conflict id.
-  DiagnoseDuplicateName(context, conflict_id, prev_id);
+  DiagnoseDuplicateName(context, name_id, conflict_id, prev_id);
 }
 
 // Returns an instruction that has the specified constant value.