use React.Children.count for counting children (#28991)

Summary:
just a minor change - uses the [count api](https://reactjs.org/docs/react-api.html#reactchildrencount)

> Returns the total number of components in children, equal to the number of times that a callback passed to map or forEach would be invoked.

## Changelog

not needed I think
Pull Request resolved: #28991

Test Plan: tested locally

Differential Revision: D21794081

Pulled By: TheSavior

fbshipit-source-id: bf6d11b2bc854d938aed7268911f89a00bb3f596

Author: vonovak

Libraries/Components/TextInput/TextInput.js

@@ -1095,8 +1095,7 @@ function InternalTextInput(props: Props): React.Node {
     const style = [props.style];
     const autoCapitalize = props.autoCapitalize || 'sentences';
     let children = props.children;
-    let childCount = 0;
-    React.Children.forEach(children, () => ++childCount);
+    const childCount = React.Children.count(children);
     invariant(
       !(props.value && childCount),
       'Cannot specify both value and children.',