Don't call valueOf during string 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 uses String.concat (like Babel does) instead of `+`, which
avoids this problem.

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 ''.concat((value: any));
 }
 
 export function getToStringValue(value: mixed): ToStringValue {