Summary:
I found the TextInput can't control input length when default value's length > maxLength.
for example：
1.Set the value in special cases
```
<TextInput value={'12345678'} maxLength={6}/>
```
2.Quickly press the keyboard with multiple fingers
```
// RCTBaseTextInputView.m
……
if (_maxLength) {
NSUInteger allowedLength = _maxLength.integerValue - backedTextInputView.attributedText.string.length + range.length;
if (text.length > allowedLength) {
……
```
when value's length > maxLength,the allowedLength not a negative number.it was transformed into a big number,because it is type NSUInteger.so the `text.length > allowedLength` always false.
[iOS][Fixed] - fix the TextInput can't control input length when value's length > maxLength
Pull Request resolved: https://github.com/facebook/react-native/pull/23545
Differential Revision: D14146581
Pulled By: cpojer
fbshipit-source-id: f53b1312ae55fad9fc10430ab94784c1a9ad4723
jsfu
f08c94bd36