Provide locations for indirectly imported instructions. (#3811)

This replaces all invalid node IDs in import_ref.cpp with references to
the imported instruction.

This splits out ReplaceInstBeforeConstantUse into a separate function
when the LocationId is replaced, as for splicing. That's the less common
case, whereas others would need to provide the LocationId in order just
to not change the value.

toolchain/check/handle_function.cpp

@@ -198,8 +198,7 @@ static auto BuildFunctionDecl(Context& context,
   }
 
   // Write the function ID into the FunctionDecl.
-  context.ReplaceInstBeforeConstantUse(function_info.decl_id,
-                                       {node_id, function_decl});
+  context.ReplaceInstBeforeConstantUse(function_info.decl_id, function_decl);
 
   if (SemIR::IsEntryPoint(context.sem_ir(), function_decl.function_id)) {
     // TODO: Update this once valid signatures for the entry point are decided.