Convert to string before concatenation

The new [Temporal](https://github.com/tc39/proposal-temporal) date/time API 
(currently stage 3) will throw when calling `valueOf` on instances of most Temporal types.
This behavior breaks react-dom's rendering of Temporal instances because the
current implementation relies on the `+` operator which calls `valueOf`.

This commit ensures that values are converted to strings before concatenation
is attempted with `+`.

packages/react-dom/src/client/ToStringValue.js

@@ -19,7 +19,7 @@ export opaque type ToStringValue =
 // around this limitation, we use an opaque type that can only be obtained by
 // passing the value through getToStringValue first.
 export function toString(value: ToStringValue): string {
-  return '' + (value: any);
+  return '' + String(value: any);
 }
 
 export function getToStringValue(value: mixed): ToStringValue {