Unclosed string literal error on regex string

Hello, I found this issue when updating the formatter version in googleapis/java-spanner

```java
package com.example;

final class Test {
// Error: unclosed string literal
String breaks = "'\\d{2}):(\\d{2}):(\\d{2})(\\.\\d{1,9})?)([Zz]|([+-])(\\d{2}):(\\d{2})))|(READ_TIMESTAMP)[\\t";
// Works
String fix = "'\\d{2}):(\\d{2}):(\\d{2})(\\.\\d{1,9})?)([Zz]|([+-])(\\d{2}):(\\d{2})))|(READ_TIMESTAMP)[\u005Ct";
}
```

Produces the following error:
```
hi on] diegomarquezp:java-spanner$ java -jar ~/.library_generation/google-java-format.jar --version
google-java-format: Version 1.26.0
[hi on] diegomarquezp:java-spanner$ java -jar ~/.library_generation/google-java-format.jar test.java
test.java:7:13: error: unclosed string literal
String fix = "'((STRONG)|(MIN_READ_TIMESTAMP)[\\t ]+((\\d{4})-(\\d{2})-(\\d{2})([Tt](\\d{2}):(\\d{2}):(\\d{2})(\\.\\d{1,9})?)([Zz]|([+-])(\\d{2}):(\\d{2})))|(READ_TIMESTAMP)[\u005Ct ]+((\\d{4})-(\\d{2})-(\\d{2})([Tt](\\d{2}):(\\d{2}):( \\d{2})(\\.\\d{1,9})?)([Zz]|([+-])(\\d{2}):(\\d{2})))|(MAX_STALENESS)[\u005Ct ]+((\\d{1,19})(s|ms|us|ns))|(EXACT_STALENESS)[\\t ]+((\\d{1,19})(s|ms|us|ns)))'";
^
[hi on] diegomarquezp:java-spanner$
```

Note that `String fix` doesn't have the error after replacing these two specific double backslashes `\\` with the hexcode version `\u0005C`.

**update**: simplified reproducer code.

從 issue 中簡化的 Java 重現程式開始，使用文件中的 java -jar 命令執行 google-java-format 1.26.0。追蹤格式化程式如何解析包含 regex 反斜線的字串常值，並確定為什麼有效的原始碼會被回報為未閉合的字串常值。重現程式能夠在沒有該錯誤的情況下完成格式化，即表示完成。

由索引模型根據 Issue 內容生成。