"source" comment in generated code can produce files that don't compile

This line https://github.com/google/closure-templates/blob/5aef0db2fdecb92cb1e3d2e926cacb5dc3923a00/java/src/com/google/template/soy/parseinfo/passes/GenerateParseInfoVisitor.java#L349

can lead to files that don't compile on windows because they'll have an illegal unicode escape.

For example, if you have a file called `utils.soy` and run `SoyParseInfoGenerator` on it, it generates code like this:

```java
// This file was automatically generated by the Soy compiler.
// Please don't edit this file by hand.
// source: C:\XXXXX\src\main\resources\soy\utils.soy
package xxx;
```

You can put a unicode escape anywhere in a java file, including comments. So trying to compile the above leads to:

```
UtilsSoyInfo.java:3: error: illegal unicode escape
// source: C:\XXXXX\src\main\resources\soy\utils.soy
^

1 error
```

Introduced via b0b39f9af2fed78333ddb7963266e9d2db3df094

This would affect any soy file with a path segment that starts with a `u` on Windows.