When diagnosing name used before declared, set the location of the usage (#4860)

Done by adding a poisoning location for each poisoned name.
Part of #4622.

toolchain/check/handle_function.cpp

@@ -255,7 +255,8 @@ static auto BuildFunctionDecl(Context& context,
   }
 
   if (name_context.state == DeclNameStack::NameContext::State::Poisoned) {
-    context.DiagnosePoisonedName(function_info.latest_decl_id());
+    context.DiagnosePoisonedName(name_context.poisoning_loc_id,
+                                 function_info.latest_decl_id());
   } else {
     TryMergeRedecl(context, node_id, name_context.prev_inst_id(), function_decl,
                    function_info, is_definition);