Invalid regex generated with 'useUnicodeFlag:true' for regexes with `-`

Reported in https://github.com/babel/babel/issues/7891

Reproduction:
```
const pattern = '-';
const processedPattern = rewritePattern(pattern, 'u', { useUnicodeFlag: true });

// throws
new RegExp(processedPattern, 'u');
```

Looks to be an issue because `IdentityEscape` is essentially redefined when `U` is enabled. https://www.ecma-international.org/ecma-262/8.0/#prod-IdentityEscape
