[SPARK-15165] [SQL] Codegen can break because toCommentSafeString is not actually safe

## What changes were proposed in this pull request?

toCommentSafeString method replaces "\u" with "\\\\u" to avoid codegen breaking.
But if the even number of "\" is put before "u", like "\\\\u", in the string literal in the query, codegen can break.

Following code causes compilation error.

```
val df = Seq(...).toDF
df.select("'\\\\\\\\u002A/'").show
```

The reason of the compilation error is because "\\\\\\\\\\\\\\\\u002A/" is translated into "*/" (the end of comment).

Due to this unsafety, arbitrary code can be injected like as follows.

```
val df = Seq(...).toDF
// Inject "System.exit(1)"
df.select("'\\\\\\\\u002A/{System.exit(1);}/*'").show
```

## How was this patch tested?

Added new test cases.

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>
Author: sarutak <sarutak@oss.nttdata.co.jp>

Closes #12939 from sarutak/SPARK-15165.