make tok stable under multi-thread

src/checker.cpp

@@ -2050,18 +2050,26 @@ gb_internal bool redeclaration_error(String name, Entity *prev, Entity *found) {
 			// NOTE(bill): Error should have been handled already
 			return false;
 		}
+		// NOTE: the insertion order is a race between the files of a package, so anchor on the
+		// lower position to keep the diagnostic stable
+		TokenPos lo = prev->token.pos;
+		TokenPos hi = pos;
+		if (hi < lo) {
+			lo = pos;
+			hi = prev->token.pos;
+		}
 		if (found->flags & EntityFlag_Result) {
-			error(prev->token,
+			error(lo,
 			      "Direct shadowing of the named return value '%.*s' in this scope\n"
 			      "\tat %s",
 			      LIT(name),
-			      token_pos_to_string(pos));
+			      token_pos_to_string(hi));
 		} else {
-			error(prev->token,
+			error(lo,
 			      "Redeclaration of '%.*s' in this scope\n"
 			      "\tat %s",
 			      LIT(name),
-			      token_pos_to_string(pos));
+			      token_pos_to_string(hi));
 		}
 	}
 	return false;